Err.. sure there is. You can change a desktop. CPU bad? Replace it. Video card outdated? Replace it. Plus laptops are always going be slightly behind in power. What's the fastest laptop processor? Now what's the fastest desktop processor? They'll never catch up because they have to balance consumption of power with power. Plus with more power comes more heat (another problem when you have such a small space to deal with).

You can't do that with a laptop. Well I suppose you could if you knew soldering and where to get replacement parts and all that. You may not need the power of a quad core processor, but someone else might. And that's something a laptop can't do.

Well I suppose when affordable laptops with quad core processors come out, desktops will be coming out with 8 cores -_-.