The problem that I see with most soft solutions is that, on the whole, VSTi designers don't PRIMARILY design VSTi's for live use. First and foremost they are a studio tool, for integration with DAW's, etc., and often the live use is not prioritized. There's a LOT of things that have to be just right for a soundset to work well as a live stage instrument, not to mention all the other things it needs for arranger integration (how well does it handle 'late' note corrections, for instance?).

How evenly are all the sounds balanced? Can you go from one horn section sound to another without having to make a volume or EQ adjustment? Does one drum kit map EXACTLY onto another, do all the bass sounds respond the same way to velocity? Can you change from a flute sound to a synth sound without having to grab a fader?

There's a LOT more to making a coherent soundset that requires little or no input from the user live than many give credit for... And, I think at this point in time, it is why the software arrangers still struggle to be as easy to use as a closed one.