I run a maxed out last-gen iMac. 3.4GHz i7, 16GB RAM, 2GB VRAM. I still run into occasions where things slow down, mostly when I'm running a zillion things at once. It's not one app that needs 12 cores, it's the overall multitasking experience.
You also resolved your own statement when you mentioned massive GPU parallelization. The Mac Pro is the only Mac to use desktop grade graphics (iMacs have to use mobile chipsets due to thermal constraints), and it's the only Mac where you can throw in more than one card.
Unless someone comes out with a Thunderbolt PCI-e adapter for desktop graphics cards to plug into, there's still going to be a need for an expandable tower workstation machine.
You also resolved your own statement when you mentioned massive GPU parallelization. The Mac Pro is the only Mac to use desktop grade graphics (iMacs have to use mobile chipsets due to thermal constraints), and it's the only Mac where you can throw in more than one card.
Unless someone comes out with a Thunderbolt PCI-e adapter for desktop graphics cards to plug into, there's still going to be a need for an expandable tower workstation machine.