Depends on what sort of multiboxing, nothing game changing for sure. ... http://www.wowpedia.org/Guild_perks#Guild_perks ... if your just doing pvp not much.

Increase Justice/Honor/Conquest/Experience points gained, gold looted and cost reduction by 10% are nice. Durability loss and cauldron buffs might be more of a benefit than it ever was with heroic attempts.

But yes, your not going to be able to get one of every race of every class to honored rep in the guild and be able to say it was worth it. i am sure there are tweaks upcoming to smooth out the achievements and the rewards a bit more.

That said, a guild for just your own use is outstanding. I love having a bank tab to trade with versus between characters. One of the reasons I am not too fond of having all the slaves in a 'real' guild. But I see that having the slaves really contributes to the guild xp so ... meh why not.