I can think about a clan with lot of strong people that do not participate. Only lower level do. The war could be easier to win. Then when they recrute they keep donations (deleted if you go out and in) so they can find better members.
Finally, donation stay, wars are won.
However I think this has minor impact and the idea is great. In our clan we have the same issue and people go out, then back in, then you need to re-promote them. Is quite a pain.

