Real-time shading/rendering is essentially a massively parallel operation -- the exact type of problems that GPUs (and supercomputers in general) are good for.
In fact, many of the top supercomputers today use GPUs.
Yep. And it would probably be good for altcoins indeed. It just can't be a good bitcoin miner because the ASICs are already out there. If an altcoin can be mined with an ASIC (I believe the purpose of some, like litecoin, are not to be?) then it is near pointless to mine with a GPU (unless in a pool).
Also, I would assume the reason behind supercomputers not being good for gaming is that they are parallelized in ways other than the chips on the GPU (entire machines are networked together via various interfaces). The software behind distributing the processing between several machines, or whatever aspect makes it super, is probably what limits the ability of supercomputers to run video games :0
The best performance/watt altcoin miner is the NVidia 760 right now, although the R9 290x has a case for being the best well-rounded altcoin miner (Once you factor the fact that you'll need 3 or 4x the 760s to keep up with a single R9 290x).
In fact, many of the top supercomputers today use GPUs.