Well yes, there is that.

But I would say, it's the neatness factor _and_ nothing else. I really, really hate the idea of having to throw out an entire computer just because a monitor has gone bad, or having to throw out a monitor just because a CPU has gone bad. I'm writing this message right now a wonderful old Viewsonic VP201b that has survived the passing of a Dell workstation, an HP Slimline PC, and has now been attached to a Mini for nearly half a decade. And I see no reason to throw it into the garbage any time soon...
Anyway, in short, I know there are some advantages to an all-in-one machine, but I'm never going to buy one.