I'm mainly a guitar player in these days even though when I seriously started thinking about music was a drummer. I've been at quite a few different instruments including having studied classical percussions such as Timpani, Xylophone and Marimba. Learning to play the actual instruments of the music I write has enabled me to write more realistic parts (at least I think so) and has probably help me speed up my writing and workflow.
Basically I think that any additional skill will help you out in your quest for gigs. Sometimes I was the right guy for a certain job simply because I was that blues guitarist who could also write ambient music.
I don't think you will get many gigs without actually going out there putting your reel around and making quite a few telephone calls. I have been lucky time to time when someone, such as a friend of a friend, has been in need for music which has also given me a few gigs. Most of the time, however, it's been me sending out demos.
The more you learn the better. The next thing could be to take care of all you've learnt and learn how to use that in order to get you that gig.
As a violinist, first of all this has helped me to be patient while working with samples. So I think yes, I benefit from it.
The other thing is that since I studied violin for years this shaped my "inner ear to result" chain. Little explanation: from childhood on I was positively surprised by the fact that my playing of this month was better than last month, and that kept me on practising. But after a certain point in my study I began to listen to myself more objectively, and moreover to project my inner ear in advance. It was like "oh, it possibly could sound like what I imagine right now, and the sky is the limit" and let the fingers do the work. This is something I try to use with samples, too.
I have been mixing in live violin tracks into my mockups now and then, sometimes with very good results, sometimes they pointed out the lesser quality of the sample work.
Having played a little bit of guitar and a little more electrical bass in my youth I seriously consider taking guitar lessons for using it in the studio. I would love to play a good rhythm guitar now and then.
The only official lessons I have had is drumming but along the way I taught myself basic keyboards, guitar, and bass. I love being able to use live instruments in my pop songs and not have to rely on samples. My guitar chops are not Van Halen like but I can throw in some nice rhythm and some mondo power chords. Same on the bass. Ironically I own a set of Vdrums but have yet to use them in my recording opting for EZ drumme and Stylus. (it's like cheating) I do have good finger skills when it comes to laying in a drum part and I find my music to be very rhythmically orientated. I wish I had learned a tonal instrument when I was young (drums just go boom) My ear is not where I would like it to be but it is getting better.
"Every time you play a wrong note God kills a kitten."
I play the guitar and played it for many years, and also learned to play keyboards... I rarely, if ever, use real guitar on soundtrack material... Takes too long to record well and you can get a guitar synth that in the mix will sound like a guitar...
I think what will help you is to really practice writing and scoring music... Having played an instrument well has helped me get a greater understanding of music and an experiential knowledge of what music is... But knowing how an orchestra works is a whole other ball game...
About the only way I could think it would make you more "successful" is if you pursue playing gigs and that helps you network with people who may want a score in the future...
Reading tradivoro's anwer - I second writing and scoring music is the best training.
Sametime I am thankful that I have played in orchestras from hobbyist to professional over many years and in all positions from last chair of second violins to concert master. Sitting two meters away from a viola section or the woodwinds is a great experience I don't want to miss, and it influences my writing for sure.
If I would not have been there and started to work as a composer I would try to attend at least some rehearsals of a real orchestra, sitting right in the middle of the orchestra with a score on my knees. Try it, it is unbelievable.
Honestly, Pfraser, I think telling people they should hire you because you can save them money by playing things yourself may work against you. Money matters, but the most important thing is your music, so if it helps the 'music' sound better on your demo and on your productions, then I would agree that -- for the purposes of producing a demo only -- having live players is indispensable.
By contrast, if you are trying to sell yourself as the guy who can give the best quality for zero money because you can play all these instruments, you risk looking desperate and, ironically, getting only the jobs where money, not quality is the most important to the director or producer. In a low budget situation, you already will have to compose the music, perform a lot of it with electronics or samples, mix it, master it, sync it, create your own timing notes. Yikes! Are you supposed to be the orchestra too?
That said, Mark Isham plays trumpet on many of his scores, including some that sound like they were done basically 100% on synth and solo trumpet. A single instrument on a bed of pads? Magic, for some films.
Even if you have very little money you may be glad to have professional players on the score. My first feature had a budget of $1,500 (one thousand five hundred dollars) and I think I spent a little more than all of it recording four cellos, a guitar, and a saxophone (separately, of course!) and that score still has bits that sound very good, even with older samples on the drums and other instruments. The reason is not just that they were real instruments, but that the players genuinely brought something to the production -- their many years of experience, artistry with the instruments and so on. It would be impossible to replicate that with a single person who's had only a year or so on the instrument.
The best way (in LA at least) to get jobs is not to get stuck on the quality of the music, which, unfortunately, is rarely the main question in low-budget producers' minds. Naturally, there is a minimum standard, but you might be disappointed to learn -- if you haven't already -- that many of the new directors can't really tell the difference between 'ok' and 'very good.'
They are looking for something interesting, sure; something emotional, but in my experience, the way to get work is to go to parties, go to events that attract 'directors and producers' not composers. Film festivals, and what not can be great. Retrospectives of someone's work, if there is a reception.
Practising 10 different instruments will normally produce at best an amateurish result, even if you are very talented (which, presumably, you are!). If you want to get work, use the practise time to go get drunk with a director instead! It's a much better way to get hired. The project I'm on now is because I played tennis with the director.
Also, directors like to go to the movies. Call one you know and go to the movies, or go hang around the production office of a student film school and drink coffee with somebody.
while I agree with JohnG that playing many instruments won't help you at all to get more gigs (if this was the original question), it DOES help you write better mock ups and THIS can help you get more gigs. But of course you can write the best music of the world, and you still get no gigs, because this is just different procedure : making great music / getting gigs.
So to get back to the instruments, I find it very helpful to know each instrument a little bit, or at least know one instrument of a family. This doesn't mean I use them in my mock-ups all the time, but it helps me to "think" more appropriate for this instrument.
I just got me a tenor trombone to get more familiar with the sound of brass. I am learning it for myself, trying to produce notes and see how it works. It is a lot of fun (for me, not for those around), and it definitly helps me to put myself in the place of the player. I play serveral instruments : flute, piano, guitar, violin, viola, serveral ethnical instruments (in exemple er-hu, duduk, a little taiko drum), organ, electric guitar, bass, trombone, yes my studio is crowded with instruments, but I'm in great company .
So, I would encourage anybody to get familiar with as many instruments as possible, to get closer to the essence of the instruments that we use in "sampled form". No need to be a virtuoso.