It's interesting how many of the commenters are starting from the assumption that this is both intentional and desirable (or in some other way smart on the part of YC) and then reasoning only on that basis. (Or maybe it's not interesting, since this is on YC's own site... :-P)
Concluding that it's not intentional depends on the premise that they don't know they're doing it, which seems unlikely.
YC seems to have a spray-and-pray approach, and it used to be run by Sam Altman who has repeatedly failed upward, so I think it's very reasonable to assume this is either not a conscious strategy or it's just a bad strategy.
Either way, the VC's value is to be able to predict whether Dropbox or Zumodrive deserves their bet, and they clearly couldn't.