The dealer is talking about replacing worn parts, while the regular mechanic is actually suggesting you wear the part a little more, which you don't want to do. I guess the mechanic has the equipment to turn rotors and it's not getting a lot use these days. I side with the dealer.
The reason you want a certain amount of thickness in your rotors is that the rotors absorb most of the heat created during braking, and dispels that heat into the air while you are not braking. Rotors are heat sinks. Brake pads are composed of materials that tend not to absorb heat. Depending on what kind of roads you travel, this can either mean very little to you or become a life and death situation. If you tend to brake very little, and for short periods, on relatively flat roads, rotor thickness is not so big a deal. If you routinely travel very steep roads (downhill) and need to use lower gears and need to brake frequently and for long periods, you want nice thick rotors and pads. Very hot rotors cannot provide enough friction to slow a vehicle on a steep decline.
I get it that you mean by your question, do you need to turn or replace the rotors if they are not scored but are getting thin. The answer is no to turning the rotors. Rotors are cheap enough to just replace them new if they worn or rusted or warped, less than $100 for a pair. The answer is yes to replacing them if they are thin. Since you are asking about rear brakes, one thing to check is whether your emergency brake is holding the car in place on an incline while the car is in neutral. You need that to pass inspection.
The other (more important) reason you want to replace them if they are too thin is that the rear brakes need to work just as well as the front brakes in order to ensure even braking front and back, which prevents the front brakes from engaging the ABS too soon during sharp or wet braking.
In most cars, the rear rotors are the easist to replace, but you have AWD, so I guess that the replacement will be as much work for you as the front rotors would be. But if they don't seem as important as the front rotors, be assured they are worth the effort and extra cost. Rear brakes need to work in concert with the front brakes.