Setting aside the ethical questions, because I don't really care, when I look at the probable outcomes of this, I think it's wise for Google to point out what's going on here. This string of stories positions Google as the smart, sciency search engine, and Bing as a collection of hacks. This is how I'd want the public to perceive the battle if I were Google.

But even though this makes Google look good, PR-wise, Bing should still use this trick, if it makes their search results better. It seems like a short term solution, but a good one to get their results more competitive, while they work on the core problems Google has already solved. Google should call them on it and expose their hackery, so people know where the good search science still comes from, but Bing should still do it. They are both playing the game very rationally.

As an aside, I don't buy the arguments of "they shouldn't be mentioning Bing." This isn't like the POTUS running against some no-name congressman - this battle is already well-publicized, via hundreds of millions of dollars of ad buys by Microsoft, so the general public already knows there is a competition between Bing and Google.