I think it might be nice if surface trees actually sold for wood, rather than just... I dunno, vanishing. Would give an early fort a boost.
Anyway, fertile lands - those in the early maps - can be farmed, but you still need a fertility stone, and your Dwarves will not visit growing things outside of the fertility stone's zone of influence... Which can be handy, as this lets you set up attractive orchards of Autumn Apple Trees, or whatever catches your fancy. Once you get into desert and snow, though, no easy surface farming.
I'd still like to build on the surface, though, if only so I can build a Dwarven Skyrim when I get to the snows.