Normally most consumers don't need a Mac Pro, even most power users may not need the kind of power endowed by the machine.
If you are not using specific applications to make full use of the dual GPU and also the 12 cores, they are basically there just wasting electricity.
Only some particular usage will fully benefit from the power of these machines
1) Video encoders, they can install a couple of such machines performing farm encoding via Qmaster in Final Cut Pro
2) 3D rendering, and design workstation. For design workstation and proofing, having the dual GPU and large number of cores will be very useful. A few of such machines form a farm to perform rendering is also very attractive
3) Simulation via HPC will also benefit alot from such systems
I personally don't feel graphic designers will rip much from such setup because most time spent by graphic designers are design time, not computational time. I suppose a powerful MBP will suffice; However having a MP will bring them more burst power, just I suppose most power will be under utilized.
If one really want to make good use of such machine, the application would be best to support OpenCL effectively to utilize the computing resources across the CPU and the GPU.