The only valid argument I can see for a new site is that the current division between 2 sites (or 3, if you count MO, although most of the quantum stuff is not pure math and wouldn't make a good fit with MO) splits QI people potentially interested in participating. In principle, this could be a reason for creating a separate site - however, at this points I see at least two counterarguments:
a) Take the union of sets of quantum people participating in TP, cstheory and MO. This number is simply not big enough to support a separate site. So the only solution would be to recruit new members that don't participate in SE model at all, which brings us to...
b) If somebody doesn't participate now because "blah, if there is no separate QI site, I won't bother mixing with physics or CS crowd. Either a separate site or no contribution from me at all", I dare say he would not make an active community member anyway, no matter what the site is. If I want quantum questions/answers, I don't complain about lack of adequate site but simply ask these questions in whatever place seems suitable (usually, TP or cstheory). Rejecting these sites because they aren't purely quantum seems like a good excuse for not bothering to participate, not a real and well founded reason.
P.S. A plea to all people who are outspoken here about needing a separate site, yet seem (at least judging by their profiles) to have no previous experience in using SE model - please, take some time to get the feel of how SE sites work, post some questions, write some answers, "lurk more". Not having a separate QI site is not really as bad (in practical terms of being able to get high quality answers from experts) as you seem to think.
P.S. 2: Please, before you dismiss TP.SE as a valid place for QIT, give it a chance. Post QI questions, try to answer those already present. At the moment, QIT's presence there isn't very strong, but you can change that by participating.