I don't like the idea of folks being able to join an alliance only to disband it, or kick folks out (I'm sure there are folks that would love the idea just for bragging rights).
But i am ok with allowing a way to join some alliances, especially dead ones as a way to resurrect it. Perhaps there can be specific land designated alliances that anyone can try to join (available in each land). So maybe the woodcutters guild in Loreroot, but not the CoE, which is more of an RP alliance? Or the water dowsers guild in MB, but not the Knights of Bell as again that seems to be more of an RP alliance to defend a land, so folks who are not really into MB shouldn't just be able to join and leave as they please.
My big concern is to prevent a few folks going around joining everything and causing havok because they can. Some folks have a lot of loyalty build up from over the years, so if every alliance is fair game, they could simply join everyone, become the leader by default and disband it. People have quit in the past over folks disbanding alliances and with such a small player base, i'd rather not see that again.