As for the pro's. Sitting more upright allows for the rider to relax, save some energy and to breath easier. Sometimes they will sit further back on the saddle to get more hip flexor muscle and quad muscle into the pedal stroke resulting in a shorter reach. Since they are not riding fast enough that being aero is a factor, they switch over to comfort and efficiency. When they attack or hit a flatter stretch they will revert back to regular riding form.

For me it's finding the best way for each climb while trying to minimize effort and to be as relaxed as possible. In longer efforts I try to use only the core and legs, putting my hands of the top's assures that I'm not using the upper body muscles.