It is telling, but it is not about soft factors. What that is really about is yield protection (yes, even at Stanford). Stanford does not take many people from Yale and Harvard (probably do more for California and West Coast students). Every year, a substantial amount of their class is taken from the waitlist. Waitlisting these people keeps their yield down so they only accept people that might actually take them over Harvard or Yale. Again, its a numbers game and having a low acceptance rate helps as far as rankings.
Hmmm...I don't know about that. At the Stanford ASW, I counted at least as many students from Harvard and Yale (each) as from Stanford--probably in the range of about 30 or so from the two schools alone. Harvard, Yale, Penn, Princeton, Columbia, MIT and U Chicago are all huge feeders into Stanford's acceptance pool. Moreover, a lot of them get accepted and still go elsewhere, so I don't think it's necessarily yield protection.