Iala
Can be prevented by setting a long limit to who can join in the fight as attacker. Say 3 weeks for the attacker and only the attacker.
That way attacker can't swell their ranks with temporary members from other guilds, leaving the defenders with no way to defend themselves as they can bring in such mercenaries.
Human nature is such that if there is a way to abuse something, there will be those who put forth as much effort as it takes to abuse that thing, to gain an advantage. If you have to wait two weeks before you can participate in a battle, people will swap guilds two weeks in advance. Or three weeks. Or four. Or whatever it takes.
As it is now, friendships, cooperation, deal-making, and even politics play a role in who fights or doesn't fight.
That's the basic idea of a sandbox: the game doesn't artificially restrict you from doing what you want to do, only other players can do that. If the game is going to continue to be a sandbox, artificial limitations need to be extremely few and only present to prevent abuse. However, if the game is going to move away from the sandbox model, then restrictions like this would make sense.
With so many other games telling you what to do and how to play, it's nice to have a few that let you make your own decisions.