Many libraries these days include both release and no-release versions of instruments (e.g. SISS, which I use). As somewhat of a sampling novice I ask you: Since the release-versions sound superior, when would you actually use the no-release versions? (provided you're not limited by CPU/memory)
As you have guessed, the no-release versions are simply a compromise option for those needing to save polyphony or ram. Pretty much the same reason that some libraries include 'lite' instruments which use fewer velocity splits or wider sampling intervals.
Unless they're programmed badly, instruments with release triggered samples almost always sound more realistic than those without.