I can see an aspiring lot jockey with absolutely no credentials to fix cars being assigned light maintenance work, such as wiper blade replacements or even oil changes, when the place gets busy. (Hey, it works for Jiffy Lube.) But a dealership which puts unlicensed mechanics on real repairs is treading on thin ice with the automaker as well as from a civil liability standpoint.
As for the original question: I prefer to buy and service at the same dealer, having worked in the car business and seen how different customers are sometimes treated when a dispute or borderline warranty claim arises. The in-house customers are more likely to be treated as long-term relationships, while the "outsiders" are more likely to be handled "by the book" as a one-shot transaction. That said, you do have every right to obtain warranty service at any dealership, so don't be afraid to do so.