Necro-post, but I have to say I think a lot of people might have been/be talking past each other here. The question isn't whether mainstream philosophy has useful insights to offer, the question is whether studying mainstream philosophy, i.e. "not ignoring it", as you put it, is the best possible use of one's time, as opposed to studying, say, AI research. There are opportunity costs for everything you do, and frankly, I'd say reading philosophy has (for me) too high of an opportunity cost and too low of an expected benefit to justify doing so. I don't think I'd be mistaken in saying that this is probably true for many other LW readers as well.

I don't advocate reading Quine directly, but rather Quinean philosophy. For example Epistemology and the Psychology of Human Judgment, which reads like a series of Less Wrong blog posts, but covers lots of material not yet covered on Less Wrong. (I made a dent in this by transposing its coverage of statistical prediction rules into a Less Wrong post.)

And I don't advocate it for everyone. Doing research in philosophy is my specialty, but I don't think Eliezer should waste his time poring through philosophy journals for useful insights. Nor should most people. But then, most people won't benefit from reading through books on algorithmic learning theory, either. That's why we have divisions of labor and expertise. The thing I'm arguing against is Eliezer's suggestion that people shouldn't read philosophy at all outside of Less Wrong and AI books.