These days most of big guilds already have multiple people with GM abilities and they do it the worst possible way - they share the account with GM char, so that they can buff MR etc 24 * 7.
I don't see anything wrong on this except for having to share password and account, which possibly open a door for scammers, so why not make this possible by allowing GM's to either set up multiple GM's for 1 guild, or give ability to launch guild buffs even to non-GM chars (like commander).
This would also prevent a guild from dying when GM stop playing the game, get hit by a bus etc :P