I know service does sell some cars... lots of time a big service bill pending is what gets people car shopping. I had a customer once that thought if she bought a new car she could still get full trade-in and forget about the $4k in service it needed.

As for service keeping the doors open... well most of the forums I'm on have all the dealer techs bitching about having no work to do. But I know you see it first hand all day long.