If you sell me a "Pro" machine, I expect to be able to fully utilize all its resources for as long as I need to. While I don't need full power all the time (once or twice a month I need it for more than 10 minutes), when I need it, I'd prefer it to be available for more than a couple minutes at a time and not make my laptop sound like a hair drier and compute like an Atom laptop.

Having said that, my 15" MBP is sufficient for my needs, but I'd not recommend it for anyone doing heavy numeric lifting. Or compiling large volumes of code.