I do this, I find having both to be wonderful. I do it because it was what I can afford, but I think that if I were to upgrade I'd go from a t2i to a 7d2(whenever that comes out) instead of a t2i to a second 5d2.
I love the fact that I can take an 85 f1.4 and put it on crop and have a 135 f1.4. No, it doesn't make the same bokeh as a 135f2, but it's close, and it lets in more light, and doesn't cost me an extra $850! Granted, I lose that stop that I gain in usable ISO by using it on a crop body, but for the versatility it adds, it's great.
Instead of owning a 35 1.4, 50 1.4, 85 1.4, 135 f2, I can own a 35 1.4 and use it on crop for ~50mm and own an 85 1.4 and use it on crop for ~135mm. That saves me from having to buy 2 more lenses. My 70-200 2.8 portrait lens becomes a 112-320 2.8 sports lens when I throw it on the crop body. A 17-40 wide angle can be used as an ultra wide on FF or a general purpose lens on crop.
The images may look slightly different from crop to ff, but just like anything else, it becomes second nature the more you do it.
So, while with a pair of 5d2s I'd want to own a 17-40L, 35L, 50 1.4, 85 1.4, 135L, 70-200 f2.8...
with FF+crop all I need is a 17-40, 35L, 85 1.4, 70-200 f2.8.