I'm trying to think of a time when i would load a VSTi but NOT need a new instrument, and i can't think of one. Everytime i wanna use a VSTi, i need an instrument to play it, even if it's second instance of the same VSTi.
The only problem might be if MPT can tell the difference between a VST Instrument and a VST Effect. That might cause a problem. But except for that i think this is an excellent idea. I second it!