I think what MS correctly sees is if they can drive up marketshare in search they'll revenue number will grow linearly, but their costs will grow much slower (as they're already largely all in headcount and advertising wise. The big delta would be to get the number of data centers that Google has).
I think its a sound strategy, but they're going to need to more than double their current marketshare before they break even. I think they originally thought that Yahoo would do it, but the Yahoo deal seems to have problems that are hard to pinpoint, at least for an outsider.