Your questions are interesting, because (outside of the startup world) they are based on sound logic. The basic rules of expected value don't apply to startups, because the present value is not a good predictor of future value. Some people are good at predicting the outcome (success vs failure), but nobody can get the number right ($10m vs $1b). Any investor who lets marginal changes in valuation influence his decision is essentially calculating a probability using a random number.
> The basic rules of expected value don't apply to startups
If you'd reword that as "the basic rules of expected value are difficult to apply to startups", there'd be some chance it was true :).
And yet, difficult as it may be to apply, expected value is the framework to rationally make a decision about low probability / high payoff investments. Of course, if YCombinator is already invested at an early stage (note: when the valuation is quite low), I can understand why pg wouldn't care too much about the later stage valuations. If you look at what's best for YC, it's first and foremost that companies get the money they need to succeed (an incentive aligned with the founders and any investors) but probably also that the valuations (after their own investment) be as high as possible so that more of the pie remains for later investments. This latter incentive is clearly not aligned with investors and as an investor I'd take the advice to disregard valuation with a big grain of salt.
