Shareholders may not take kindly to a boss who has turned down a fair offer for the company—although Microsoft may yet return to the fray and bid for Yahoo! at a later date.
I recall an incident in a handball game when a referee refused a player's request for a time out for a glove change because he did not consider them wet enough.
Plus, it's hard to imagine Groupon re-enters the acquisition market, after having rebuffed Google's $6 billion offer (due, in part, to a cash vs. stock issue, I'm told).