Not had an issue with this before I just think people are doing it now copying each other, and they will eventually get bored of doing it.
What if someone joins a guild but turns out to be the wrong move for them, they can't join another guild for a week or a month, they will leave the game. You have to try simulate every scenario and find the most effective but least restrictive cost. One way to prevent this would actually be to let guild alliances be more effective at wars, like 2 allied guilds defending/attacking SW yields points for the same team. Doesn't fix guild hopping for arena or gc but really arena can be boring anyway and gc requires the effort to sustain multiple level 20 guilds.