Every time I see this now I think of Digg and Reddit, both which seem to be having issues supporting themselves with pure ads. I actually think those two businesses and Quora could likely support a small(ish) staff and make enough revenues to make a decent profit, but I'm not sure that would get them into the decent exit category.
Demand media I was going to actually use as a counter example since they're not yet profitable (http://blogs.wsj.com/digits/2010/08/12/where-did-demand-medi...)
Last.fm was "hoping" for profitability this year (http://paidcontent.co.uk/article/419-interview-cbs-thinks-la...)
For the most part we're not talking public companies, so it's hard to be completely accurate. But I think it's safe to say that there are a lot of online advertising funded businesses who are providing strong returns to their partners/investors.
One of the big reasons I think investors are excited about Q&A is that the model can be seen as a better replacement for search engines for long-tail, high-expertise questions. This is probably the reason Quora is trying to keep question and answer quality as high as possible. So far I think they have succeeded, although this will always be a tough thing to balance with growth.
It also seems that Quora is aiming squarely at Stack Exchange, and both companies seem to be holding their long-term strategies close to their chest. Perhaps they see some monetization strategy that the rest of us are failing to grasp?
However, Quora probably doesn't have to stick with "pure" ad models -- and I don't expect them to. Just because they say they aren't focusing on monetization right now doesn't mean they're completely blowing it off. The Quora team is full of some pretty smart people who I'm sure know a bit about what they're doing.