A RTW specialist could give you an answer about rules about RTW tickets, maybe that's one of the reasons, don't know.
Regarding the time you get back: no, that is not the case. Because:
1. It is just a matter of hours, while your RTW trip is a matter of months
2. Just imagine yourself the following theoretical thing: if you travel around the world with a constant speed, either east or west, and you will end where you started, do you think that time matters at all? You will end at the exact same time at your final destination!
One advantage though, if you don't travel to far at one leg: jetlag will be much less, if not totally absent. 
Mike