Why is it interesting to discuss whether mainstream philosophy is "useful" in the sense you mean this concept?

I love to read and write interesting things - which is why I take to heart Eliezer's constant warning to be wary of things that are fun to argue.

But interestingness was not the point of my post. Utility to FAI and other Less Wrong projects was the point. My point was that mainstream philosophy sometimes offers things of utility to Less Wrong. And I gave a long list of examples. Some of them are things (from mainstream philosophy) that Eliezer and Less Wrong are already making profitable use of. Others are things that Less Wrong had not mentioned before I arrived, but are doing very much the same sorts of things that Less Wrong values - for example dissolution-to-algorithm and strategies for overcoming biases. Had these things been written up as Less Wrong posts, it seems they'd have been well-received. And in cases where they have been written up as Less Wrong posts, they have been well-received. My continuing discussion in this thread has been to suggest that therefore, some useful things do come from mainstream philosophy, and need not be ignored simply because of the genre or industry they come from.

By "useful" I just mean "possessing utility toward some goal." By "useful to Less Wrong", then, I mean "possessing utility toward a goal of Less Wrong's/Eliezer's." For example, both reflective equilibrium and Epistemology and the Psychology of Human Judgment possess that kind of utility. That's a very rough sketch, anyway.

But no, I don't have time to write up a 30-page conceptual analysis of what it means for something to be "useful."

But I think I still don't understand what you mean. Maybe an example would help. A good one would be this: Is there a sense in which reflective equilibrium (a theory or process that happens to come from mainstream philosophy) is not useful to Eliezer, despite the fact that it plays a central role in CEV, his plan to save humanity from unfriendly AI?

Another one would be this: Is there a sense in which Eliezer's writing on how to be aware of and counteract common cognitive biases is useful, but the nearly identical content in Bishop & Trout's Epistemology and the Psychology of Human Judgment (which happens to come from mainstream philosophy) is not useful?