Yeah, I agree, I think this is a great idea to prevent guild bank drainage.
While setting appropriate limits for ranks is a good start, it still has the ability to be abused. For example, even if I set someone to have one stack every week or something and there is a stack with 500 potions, they have the option to take all 500 potions and I really can't prevent that (besides allowing them not to take any stacks, which I wouldn't want to do anyway), and I think this is where a majority of the problem comes from. I think having an item withdraw limit instead of a stack limit is *much* more appropriate and it will continue to help with this problem.