If anything, the fact that voters have had to prove themselves worthy with the strict requirements makes a simple majority even less convincing. The reason being that, if all voters are considered intelligent and responsible enough, then any vote against banning must have an extremely good reason behind it. It wouldn't be good enough to just say 'Oh, 51% Uber vote, that's a majority which is good enough, obviously the other 49% are wrong and are idiots' because the process was designed to eliminate that idiocy, or noobishness or whatever you want to call it.
However, I can sympathize with j7r's side of the argument here, as it is next to impossible to justify any particular voting threshold from a purely objective standpoint. I'm not necessarily in favor of 2/3 exactly, just something much higher than 50%, but of course if there was 100% confidence in the legitimacy of voters, even a single vote in disagreement with the majority would be enough doubt to keep a Pokemon out of Uber or BL. I do not have a solution to this problem myself, and there probably isn't one, but I think it is an important point that needs to be made to put this discussion into perspective.