The danger in this is that it is quite possible that at ~36 P/E Google is near the upper limit of its practical valuation for the near future.  If they borrow big against their holdings then they have to pay back the loan and also service the debt.  If the share price drops then they would have to sell even more shares to cover the loan.  As it stands now they will still have such a huge chunk of voting rights that it would take near unanimity against them to overturn their decisions and in return they get big piles of cash that they can probably get a better return on by placing bets on other ventures.