I don't think it is particularly good for the media. If you think about it, you're making it go another chemical phase change cycle for no use. In fact, a full erase actually writes some sort of bit pattern that corresponds to a decoded 0 or similar, so yes - it is writing data and wasting the phase change life away (some cheap discs die eAsy). Good discs won't mind, but then again, you are wasting your laser life to do this, so i'll just stick with the direct-overwrite if I were you
Oh and I forgot - DVD+RW has a wobble built into the track for burn laser positioning. When you overwrite, and everytime you overwrite, the wobble is dulled corrupted or whatever, and it gets harder and harder to read the wobble and hence burn quality lowers :).