1) Much more ergonomical. I get neck pain when working on my Macbook for hours, looking down at the screen
2) I need two external screens. A Laptop Screen + an external Screen is again not ergonomical, pains my eyes to look from a decent 24" into the Macbook Display with its insane pixel density. I like same pixel density on both screens, and same size
3) Noise. For what i am doing the Macbook constantly screams at me through its fans (i7 quad version). My Desktop is silent when doing the same tasks
4) Glossy Screens, i totally hate them and today its almost impossible to get around them.
5) Laptop only is a compromise. A 15" is pretty huge and heavy. A decent Desktop for serious work + Macbook Air for on the go would be perfect.
6) Sync tools are so good nowadays (yay dropbox) that its pretty easy to have everything in sync across multiple machines.