Last time I saw this suggested, the more-or-less official response was along the lines of "we thought about it, but we'd be unable to convince everyone that tho

Last time I saw this suggested, the more-or-less official response was along the lines of "we thought about it, but we'd be unable to convince everyone that those types of links aren't being tampered with (ranking-wise)" from which I inferred that the trust of their community was (is) worth more than that revenue stream.