It's not just about power. I would never buy an iMac because I like being able to actually upgrade my components as they get older. With the form factor of the iMac, you get all of the disadvantages of a notebook (less harddrive bays, less ram slots, no expansion slots) with all of the disadvantages of a desktop (namely, it's not portable). It just seems silly to me.
Additionally, I hate 16:9 displays (16:9 is tv widescreen, 16:10 is computer widescreen) and I would much rather use my own displays (2x dell ultrasharps) than the ones apple makes, so building the display into the computer is a disadvantage to me.
Sigh... It's a shame really, I don't need nearly the kind of power that even the lowest Mac Pro has, but it's the only desktop mac that I could buy at this point.