I meant desktop as opposed to workstation, but it turns out a workstation is a desktop, it's just distinct from a personal computer (according to wikipedia, anyways). Perhaps my post should have read "affordable desktop computer".
Most people buy desktops rather than laptops because they are cheaper, more powerful and easier to upgrade. The gap between the iMac and the Macbook Pro is typically very narrow in all 3 of these respects (excepting the last 18 months in which the 27-inch had quad-core and the laptops did not).