Yeah, it's a tough call.
On the one hand, we really shouldn't give a feat a high score just because it's useful in certain ultra-specialized builds. "Skill Focus" is brilliant for Jumplomancers, but 90% of the time it's filler and weak filler at that.
On the other hand, I think we have to work from the assumption that the people taking feats are the people who are making use of them. "Improved Trip" is a poor feat if you don't have the Str to back it up, but we have to assume that people who take it are going to be able to use it effectively.
I think TWF falls into the latter category. "Rogue" is hardly a specialized build, and there are other classes and builds that benefit from it. I think the category of characters that benefit significantly from it is large enough to be taken as the baseline for that feat. Others may certainly choose to pick it up even if it's not as effective for them, nobody's stopping them, but I think the pricing should assume that it's being put to profitable use.