With MS, they'll be "one hardware supplier amongst many", but with less leverage and freedom to make the platform theirs than with Android.
In Android, they'd build on a known platform with success or failure in their own hands. Don't like what Google is doing in the future? They'd have options without abandoning the platform. Hell, they could let Google develop the core, port Qt, use Bing maps and search, and Amazon Appstore.
This partnership benefits Microsoft's executives in the mobile division, who will have their vindication in a major phone maker going with their platform instead of Android. Their bonuses will be outlandish this year. It hardly benefits Nokia, who is betting the company on an unknown platform nobody else is betting on.
A friend of mine usually said that when you see two lines in the bank, one being much shorter than the other, you can bet the short one is the wrong line for you.
Well.. Stranger things have happened.
To be clear (and fair to Elop), whether or not he was effective at his role wasn't very clear to me. What I can say is this:
* His time at Microsoft was quite short. While he may have a monetary incentive for this deal, I doubt he has any cultural ties to "go Microsoft"
* His impact on MS was (to me, anyway) minimal. He seemed to be in charge of keeping the ship going in a straight line, without the pressure of changing direction. There definitely wasn't anything like what is going on at Nokia to "test his mettle", so to speak.
Contrast this to something that Sinofsky did: http://www.cornell.edu/about/wired/
a bit OT but worth it nonetheless: my father always sas than whenever there is bureaucracy involved (i.e. the DMV, embassies, other government departments, etc.) one should choose the shortest line - it means that the clerk in place there is probably the most efficient and less inclined to play "power games" with you. Served me right on several occasions.