You know the best part of being the head of the mages guild? The freaking posh room in the top of the tower.
The herb-growing chest, and just a place to chill. Not that inn fees are killing me, but I feel safe leaving a bunch of inventory stuff I don't really use but don't have the balls to sell, and it's a free night's sleep.
So forget actually being a competent mage (I heal and open locks...that's 95% of my spell-casting), I'm just glad I have the room. Enchanting items comes in handy too. I can't see any of the other guilds giving me those kind of benefits (especially since I already have a sidekick, which is the big reward from I think the fighter's guild).