December would actually be awful for me, since I'll have exams in January. And I'll probably be playing the new Assassin's Creed then, anyway. February 2012 would suit me better, which is between semester at Uni here.
The thing is, TOR needs some space for people to get into it and if ME3 follows to closely to it (even if it is more than two months), it might lose customers very soon. It'll need its time to prove itself, definitely since a lot of previews have either been cautiously optimistic or pretty pessimistic. Of course, even if the time between the TOR release and the ME3 is large enough, there are other games waiting to compete with TOR in that period. But it doesn't harm to give your own product the room by not letting your other product compete with it.