Buying Quora would be a waste of money for Yahoo. It's incredibly overvalued by the market right now. There's no way I can see to monetize effectively enough to make the valuation make sense, and they aren't growing fast enough (or at all) to justify the thinking of "with all this traffic, it's gotta make money!" Their traffic plateaued about a year ago, after a short run up in the beginning when there was tons of hype.

Given that there are better (or at least more popular) options for many types of questions (StackExchange seems to have figured it out for tech questions, and they aren't saddled with an insane valuation and so have a lot of freedom to exist without even having to grow...though they are growing), I just don't see how Yahoo buying Quora could possibly serve to improve Yahoo; Yahoo's already pretty good at being second or third best in the market.