I have to disagree. Currently, the only reasons I see to go premium are the wardrobe, and (at least for now) the queue priority. The latter is probably only going to be important for the next week or three.
90% of most modern MMO's (Including this one) you level ridiculously fast in anyways. Getting an xp bonus is absolutely no incentive at all. (Heck, half the MMO's I play I'd almost be willing to pay more for an xp reduction. Playing GW2, I'm almost lvl 40 after finishing the lvl 15 areas. I strongly suspect I'll be level capped by the time I finish the lvl 25 ones.)
That said, I definitely agree you should be able to individually purchase wardrobe slot unlocks, and buying an outfit from the cashshop should automatically unlock it's slot.