There should be a 24 hour time period after leaving a guild before you can join a new guild. Its not fair to the people doing their part in those guilds to have people join, collect alliance gifts, then leave again all day every day. Also once joined into a new guild there should be a 24 hour period before newcomers can collect alliance gifts. This would help smaller alliances build members and detur others from pretty much stealing others hard work.
This is not a good idea as there are many guilds that swappers players for fort battles and such. There are also people joining friends guilds temporarily to say hello or what not. Not to mention the massive amounts of resource trade between to different players in different guilds