The flex role is nonsense, that's what it is. I was talking with a friend about this last night, it seems to work as a catch-all spot in groups... one that tends to step on toes. I've heard more than once of scenarios where the flex player has said something to the effect of "I'm going to use a charger", leaving the dedicated charger on the team in a real awkward spot.
I'll confess to not being too experienced with roles and the assignment thereof, but I've yet to hear any good experiences with Flex.