Can your virtual orchestra imitate a real one exactly?
I'm just going to pick on one example here - that of divisi. If your orchestral virtual instrument can do this, then it will sound just a little more like a real orchestra than one that cannot.
Think of a real orchestra, but just the string instruments - first violins, second violins, violas, cellos and basses. That, according to simple arithmetic, should give us the potential for five-part harmony. In real life however the basses normally double the cellos an octave lower, or provide a bass when the cellos are playing a melody line, or sometimes get let loose for a special sonic effect. (And don't forget all those bars' rest.)
Normally therefore the string section is capable of four-part harmony. But what if the composer wants more lines? Or what if he or she wants shimmering high harmonies in the violins with no other instruments playing?
Well since there are several players in each section, up to a dozen or more in each violin section for example, they can easily be split up to play the extra lines.
In musical language, this is called divisi.
With a virtual orchestra, there shouldn't be any problem. Since you can play any instrument (counting a whole section here as an instrument) polyphonically, then you can sequence as many string lines as you like.
But it won't quite sound the same.
If the first violins of a real orchestra play divisi, then each line has only half the number of players. In a virtual orchestra without divisi, each line sounds the same when played alone or played together. And when played together the level is around 3 decibels louder, whereas a real orchestra would not change.
It sounds like a small point, but an orchestral virtual instrument that doesn't have a divisi feature will sound thickened and congested compared to a real orchestra when composed for in this way.
Of course, there aren't that many listeners who would notice this. But for a composer who takes pride in his or her work, it makes a significant difference.