Well, I have read much about the NI Kontakt Playere 2.2.3, the net result being that I am as befuddled as before. It appears to me that I don't have a need for it. I am not likely to use it with Sibelius, don't see a benefit to using it with Sonar. My work will be almost exclusively GPO Steinway and GPO Organ, and the new Steinway eventually. All works well for me using Sonar and VST. I have some sound font organs, which I have not used since I got the GPO organ. Am I missing something? My reasoning is this: if it works, why fix it?
I don't think there's reason for befuddlement - you've actually answered your own question in your post.
What is helpful about KP2 for most users is that it makes doing large pieces using many instruments easier to assemble.
For me, as with other users, it's very convenient to have the one engine where I can have all my Garritan libraries nestled together, mixing and matching instruments from GPO, JABB, CMB and The Strad. On top of that, parameters for controlling the instruments are more detailed and thorough in KP2, and there are many excellent audio tools available also.
But you do pieces for solo piano or organ. Your technical needs are very modest in comparison. So to continue using the old Kontact with your single instrument, coupled with all the facilities of Sonar - you are set just fine.