The problem is, in order to know that Quora is good enough to force you to use FB Connect even though you don't want to, you have to use FB Connect... I'm certain this slows their growth.
Or maybe it makes sure that the only people who go the length to sign up are aware of the benefit of Quora, and therefore likely to be quality users contributing the the quality of Quora. In a sense, Quora uses Facebook as a filter to get great users.