The news that the leading Muslim Brother Abdel Moneim Aboul Fotouh is being expelled from the movement should come as no surprise. It's true that in doing so, the MB is losing a widely respected figure that many see as the more moderate, acceptable face of the Brotherhood. Aboul Fotouh frequently appears on television, and has influence as the head of the Arab Medical Union, a professional syndicate. He is also a leadership figure for the vocal minority of young Muslim Brothers and their sympathizers who want to see the group change with the times. But is he becoming a major thorn in the Brotherhood's side for his desire to run for the presidency.
This is not primarily because the MB feels it is too early to field a presidential candidate, even if that's part of the picture. It is first and foremost about electoral strategy and a long-term plan to increase its political influence.
Right now, the MB is fielding candidates for about 30-50% of seats in the forthcoming parliamentary elections and none in the presidential one. That may look like restraint, but it's not: it's a very clever strategy that will extend their influence beyond what it might be if they fielded more candidates (assuming they're even able to.)
By not running for more seats in parliament, the Brothers appear to exercising restraint but in effect are positioning themselves as power brokers in the races in which they don't run. Let's assume the next elections will be under the same constituency-based system as before. In many constituencies, there will be a block of voters who would have voted for the MB which will become available as a voting block to give to another party's candidate. The negotiations to grab that voting bloc will make the Brotherhood a key influence in local politics (and indirectly in parliament), because they probably represent the single largest such bloc even if they can only get a maximum of 15-30% (assuming their voters are loyal and will follow local leadership's orders to vote for another party's candidate). So say you're the Wafd's or Social Democratic Party's candidate in the Cairo district of Sayyeda Zeinab: can you really afford not to get the Brotherhood on your side, and thus owe them a political debt that they will be able to cash in at a later point (or to trade during the elections for support elsewhere?)
The same principle applies for the presidency. The Brotherhood probably does not stand much of a chance if it fields a candidate — and I think neither does Aboul Fotouh. But if he runs, he will surely get at least some of the MB vote. If he doesn't run, there will be more MB votes to offer presidential candidates such as Amr Moussa, Ayman Nour or Mohamed ElBaradei (if he decides to run.) The MB will be in the position to be a kingmaker, not because it is so strong, but because it is the largest and most disciplined part of a very fragmented political system. I don't think the basic level of support for the MB is much more than 20%, in part because they are not likely to add many more voters among the majority of the public that never voted (since they had the most motivated people under the old regime). But that fifth of the electorate consists of a big swing vote that they will use to further legitimize their new legal status in partisan politics and widen their influence over policy. The presidency is particularly open to such electoral calculations, and indeed we saw Ayman Nour court the MB in the 2005 race.
What Aboul Fotouh's candidacy does is not only weaken this strategy. In a highly disciplined movement, expelling him was the only choice possible lest others contest the leadership strategic choices and further erode their influence. And it also opens the most public schism in years at a time of real division about the orientation of the leadership.