On car lathes are only better in theory. Yes it would be optimal to cut a rotor on the hub which it is mounted however, these units are difficult to set-up correctly. The are light and as such move around a lot. Automakers push these units on dealers because they feel that with less dissassembly time a tech should be able to do a warrenty brake job in half the time. I have used three different types of these lathes and have yet to find one that worked well. As for turning rotors, once again yes this is correct in theory. If you had your own lathe and could set the depth of the cut yourself then yes turning a rotor a small amount would be fine. However, in the real world of mechanics someone is going to remove more than needed. This happens for several reasons, mostly because of time. Get em in and get em out. Rather than take several small slow cuts it is more likely a tech will take one deep quick cut and MAYBE clean it up with a slow shallow cut. Obviously not all techs are the same but....working with many I have seen few that care about how soon you will be back with another warped rotor.
I guess I agree with FP but unfortunatly have no faith in someone eles working on my car. Get new and replace.