I had a dealer change my oil for ten years. Here's the damage sustained on various cars:
Oil plug threads stripped, replaced with plastic plug.
Oil plugs stripped, plug held in with two threads.
Oil overfilled one and one-half quart.
Oil filter installed with two gaskets (old one not removed) causing leak.
Leaking oil plug, not tightened. Finger tight.
Low oil level (three times).
That's why I change my own oil. Since I began maintaining my own cars with OEM parts, I've gotten no less than 265,000 miles from my Fords -- Excursion, Expeditions, Explorers, Focus, and Taurus. When I'm under the car I get to see the general condition of things. I find other problems before they cause more harm.
For me, it pays to get off my rear end and do the job nobody else is going to do the way I do.