It depends on how often "need to be mobile" and what you want to do while mobile happens. For me it is at least a few times per week that I want to take my laptop to a meeting, client demo, finish up some work from home, on flights, and so on.

A desktop + ultrabook would be cheaper and easier to carry around - but the hassle of keeping them in sync is too much. Everything from Homebrew package versions to keyboard mappings in different IDEs to browser login cookies. A beefy laptop that stays connected to a dock 95% of the time avoids all that.