To add a slightly less technical perspective, I bought a new Mac Pro because it fits my preferred working style rather than because I need its specific computing power (And also because my 2008 MP crapped out and I got tired of fixing it).
When my old Mac Pro died, I ran out and bought a Macbook Pro Retina and used it as my desktop while waiting for the new cylinder MP. The MBPr was stunning and performed like a champ. However, I was reluctant to use it as a laptop (i.e., travel with it) since it was my sole platform for client work and I didn't want to jeopardize my ability to work/deliver should something nasty happen on the road.
So maybe I'm a bit old fashioned, but I like to have a desktop mac for my primary workstation (I'm a graphic designer and photographer) and a laptop for the road. I use a high-end, wide spectrum monitor for color critical work, so while it's true that an iMac would have the power to run the stuff I need, I prefer to have options with my monitor.
Again, just another perspective that might factor into a purchase decision. How do you like to work and what works best for your situation? It isn't always mandatory to have the fastest and latest (unless, as in my case, it falls into your established criteria/working style).
