I pay for the messy jobs, like oil changes. It's not that they're technically difficult, but I just can't be bothered hauling around all that oil, then hauling it back to recycling, etc. It's laziness as much as anything else, doesn't take a genius to put a pan under the bike, undo a few bolts and watch the oil drain...
I trust the local shop implicitly, the guys working there ride, and do serious **** like rebuilding engines etc as well as bike maintenance. It's worth the money to me. Plus, if and when I need them for some more serious work, I want them to still be in business, so paying them for routine service seems like an idea.