Well Alan some interesting points. I dont use the MIDI side of BIABand I dont use RealBand.
The addition of VST plugins has opened up a new world here

Given good plugins a lot can be done, but your point about mirroring the slight ideosynchacies of a performance is also important. If you play eight notes in a row its natural to play the first and fifth notes louder and the third and seventh also to a lesser degree (assuming its nto reggae of 3/4).
Another problem is that MIDI instruments function with note off. Some real instruments function this way, and its also possible to let a note decay, but the passage from one note to the next on various acoustic instruments is by no means clear cut. Some instruments have a muted characteristic transition at least in legato playing - I am thinking of valve instruments, some insturments like guitar have some notes that often continue (the open strings) and others that rarely do - the fingering of guitars can also vary from note to note, from ghost notes to more staccato attacks, strums sound some notes more than others, and this all adds to the character of the sound.
Each instrument has its unique characteristics in this repsect and emulating them makes for a more authentic performamce. To do this one must know how the instrument operates and also how to achieve this using MIDI and the actual sample set you have

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