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.