In short, imagine you are one of these guys, the Yahoo! Board of Directors.
And then you get this letter from S Ballmer Esq.
Now you feel like a kid at school again, right? That's quite a self-righteous, I'm-not-angry-just-disappointed tone he's got there. Wouldn't make me any more willing to help my company get sucked into the maw of the big M.
(Due respect to JG@DF for the discovery of the letter.)