
There is one thing about my old 5D that I really like about it better than my crop 1D Mark IV. For general work and 'walkabout' type shooting I really prefer how the FF sensor matches up to the 24-XX range lenses. I find 28mm (or 17mm on EF-S) to be just not quite wide enough on the wide end.
On the 1D thus I usually use the 16-35, but then the long end isn't long enough. Basically, when I want to carry one body and a zoom, the FF bodies offer a match to the lenses I most want to have with me.
I'll also point out that this point brings up a second topic relative to Magnus3D's example images above. Sure, the 1.6X sensor 7D isolates the subject with a 300mm f/2.8 lens. I think where people find FF Canon dSLRs useful for control of DOF is on the wide end.
There are precious few choices for 1.6X bodies that are both very wide and very fast. Also, a handy wide angle zoom like the 24-70 has no functional counterpart on 1.6X.
All this doesn't mean everyone needs a FF camera. Heck, there isn't even much snobbery to be found in owning one given the used 5D market. But if you look at the specific points I mention, this may match up well to some people's needs. Those folks should look at a used 5D possibly.
I agree with you and in fact it is the lens selection that draws me to full frame. Wide primes like the 24L just are not the same on a crop. Same goes for the 24 tse or in fact the very affordable samyung 14mm. Just seems the primes are designed around the full frame format.