Put simply, it really doesn't matter how overvalued something is if your primary concern is something other than maximizing your return on investment. 

Take corporate buybacks for instance. Both equity-linked compensation and the market's tendency to focus on quarterly results at the expense of the bigger picture have compelled corporate management teams to develop a dangerously myopic strategy that revolves around tapping corporate credit markets for cheap cash and plowing the proceeds into EPS-inflating buybacks. Whether or not this is the best use of cash is certainly debatable but when the goal is to manage earnings and appease stockholders, that doesn't matter, and indeed, companies have an abysmal record when it comes to buying back shares at levels that later prove to be quite expensive. 

In America, the price insensitive corporate management bid simply replaced the monthly flow the market lost when the Fed - the most price insensitive of all buyers - began to taper its asset purchases. Of course QE in all its various iterations playing out across the globe, is price insensitive buying taken to its logical extreme. With the ECB's PSPP for instance, limits on the percentage of an individual issue that NCBs are allowed to own apply to nominal amounts meaning that, to the extent NCBs can buy bonds at a premium to par, they can effectively buy fewer bonds than they otherwise would have and still hit their purchase targets. In other words, if you overpay, it's easier to stay under the issue cap when supply is scarce in eligible paper. So in some respects, the more EMU central banks pay for the bonds they purchase, the better.