I agree with Plutoluvr. We've had great to mediocre service, but I've never noticed any decline in service when we were on the dining plan.
I'm sure it's more of a restaurant thing, or an individual server difference, than something having to do with the method of payment. We see patterns in the service from restaurant to restaurant, both in the quality of the service and the overall interaction with the waitstaff -- but we see it consistently, no matter how we pay.
The only difference we've seen with the DDP is that many servers take extra time to make sure we know what we're entitled to, and also to offer special suggestions. In that sense, I guess I could say we've gotten better service with DDP than without.
I can't imagine a server "taking it out" on customers because their restaurant is not going to be participating in the DDP the following year. I can think of a lot of things a server might "take out" on customers, but that's sure not one of them.