Its not that valleys are a big deal, but its just not fun in having to recapture them, and having them set back to lv.1 is a problem, especially if your alliance has all the others in your area. Lakes are the ones that are the most important for lv.10 cities. If you are still green in food production then you are doing something wrong.

Valley stealing is a tactic, but not one any player wants to be on the receiving side.