I recently built a desktop computer for less than $500. To get the same performance out of a laptop would have cost $1800+. I almost never travel, and almost never need to use a computer anywhere other than at my desk. If you're going to have a full-size monitor, keyboard and mouse, why triple or quadruple the cost of your system by getting a laptop that you don't need?
Besides, laptops (last I heard) simply don't last as well as a desktop. I'm interested in systems that I can build and use heavily for 3-5 years minimum. I just don't care to migrate data any more often than that, etc.
That said, I've got a laptop too - but never as a replacement for my desktop.