I agree that getting 100s of people to link to LessWrong with the anchor text "rationality" is unlikely to provide much of a benefit (though, hey, it might -- search engines are a big black box), but LessWrong is a reasonably well-trusted site (2k backlinks, most of them quite high quality, see here); having 10s of links (and given how much emphasis Google is meant to place on anchor text at the moment), it could give a substantial boost at the margins.

IMO, I think a better question to ask is how many people are searching for the search term "rationality"? Seems like a weird thing to search for.