I think the best philosophy is that non-combat trees should (and mostly do in D:OS) benefit combat in indirect but mechanical ways. Say you spend a bunch of points on Barter so you can buy better armor, more potions and arrows, etc, and thus do better in combat because of that. Not that the economy is really balanced so that barter offers you any real benefit except in early game, but the idea is there. Pickpocket and lockpicking should let you get some nice extra loot for spending the extra points, but they really aren't worth it as is.
If all you spend your points on is straight fighting skills, you should feel like you're missing out on certain item, non-combat quest solutions, and EXP benefits, not just some flavor. I expect the big patch will address some of the lack of benefits with some of the non-combat skills.
