Just my two cents to this...
There is no need for a lot of different models for vegetation. You get a bunch of variation by using different rotation, size, color and animation state - probably even changing the geometry on the graphic card itself. All these things can work with just one model. And some tropical-shooter-thingy-game proofed that it works pretty well. Didn't they use just 7 or 8 different models?

Personally I need no instancing in my projects, but if someone knows, how it's properly used, there can be huge speed advantages.