I guess my objection to this would be that i've seen it executed poorly by many MMOs. I would be so bold as to bet that shortly after "vanity slots" would be implemented, SE would begin making armor sets that are much like the wedding garb; completely useless. It wouldn't even be much of a stretch to say that they would charge real currency for these in-game items, much like other games that won't be named.

I don't want to say that the idea isn't a cool one. But, one staple of FFXI has been "town gear": a set that is pleasing to the player aesthetically, but has almost no utility.