MIDI samples with fixed ADSR.
How ya gonna do slurs, hammerons or pulloffs with that?
Not to mention the virtual myriad of other more subtle things that an accomplished player of any instrument can do? That includes Percussion instruments. Three, four or even six Velocity Layers can't cut it, although more is better for helping create illusions.
Let's face it, ANY of the currently available artificial solutions consist of tradeoffs, be they MIDI based, Loop based, "RealTracks" based or the like.
Realtracks does solve the ADSR problem, though, which is a biggie IMO.
The tradeoffs are rather obvious.
I view all of this as the creation of
illusion -- simply because it is the only possible outcome given the inputs available.
Be pragmatic about it. Do what you can do, not what you wish it could do or frustration levels will be higher than the whole thing is worth.
IF you can't live with the artificial solutions available today, the alternative is easy enough: find real talent and record them. If you don't know what that is like firsthand, try standing in a cold shower fully dressed while ripping up hundred dollar bills for starters.

--Mac