If you're president and you're incapacitated for hours or even days because of health issues, it's problematic. I don't know if President Bachmann (yes, I know that will probably never be the case) could make fast and important decisions during one of these migraine attacks, and being able to do that whenever it is necessary is essential if you're president.
The point: the Presidency is the ultimate mind job -- and migraines could incapacitate someone as completely for a day or two as a stroke.
Just imagine how badly things could have go in a disaster, as when Saddam Hussein invaded Kuwait, had the President at the time (the elder Bush) had a stroke. A rapid response can be essential.
