The Mac Pro hasn't seen a case update in at least six years. What other Apple product has been neglected like this?
Personally, I wouldn't be entirely surprised if Apple was just keeping the Mac Pro alive long enough for third-party Thunderbolt accessories to catch on. Once you can plug in all the expansion you needed over Thunderbolt, what's the point of the Mac Pro? Just faster CPU and more RAM -- so maybe they'll release a special Mac Mini that's like a normal Mini but with Xeon processors and a ton of memory.
They have a "Mac mini server" today, and they've had an "Xserve cluster node" in the past, so it wouldn't be completely out of line for Apple to release a special version.
A lot of us wouldn't find it ideal, but it could be an answer to the "I need a Mac with 32GB of RAM" question, and Apple does seem to prefer all-in-one boxes that users plug their own external accessories into.
What, besides A/V work, is a 6+ core OS X machine necessary for?
Not that you can't run these on smaller devices; these are just easy ways to use up a lot of CPU.
When I was working on quantum state diffusion, it took many hours on a 24-node cluster for a single run. In many of these tasks, the problem will expand to consume all reasonably available resources; more cores allow greater precision, wider sampling of parameters, higher fidelity, etc.