I have a neat use for governors actually. I take one with me when I'm going about capturing enemy cities and it helps get the city back to full production right away without having to wait on unrest-reducing buildings.
What this means is that I can be training new troops close to the front lines right away, especially if it's a fortress I took and all the troop buildings are present already. Useful if I'm fighting a bloody campaign or my existing troops just don't cut it. I mean yeah, I could just march new troops all the way from back home, but sometimes you just don't want to do that, say if you need to fortify the main part of your empire against attack as well.