I agree that banning any political party is not good. The problem is that the militias interfere with the elections, attack other candidates and their supporters, intimidate people and even force them to vote their way. I guess the solution would be for the state to be able to handle the militias and enforce order. But the state forces are still rather fragile. So they Iraqi politcians opted to try to deprive the militias from political cover. I am not sure if this would work either. Whichever way, I think that Sadr and his supporters would still be with us for a while, unfortunately.
I think this mirrors the technique that Syria and Iran are using in Lebanon....