Thing with Kaho/valk customs is that most people opt for 1 custom at any given time. The thing with costumes is - people collect them like hotcakes. They can be given out in events, quest rewards, AD shop. Mostly costumes do not have any stats or properties other than looks but sometimes they do. So this slot tends to act as an extra slot too. In iRO, they often give costumes with expiring stats. For example, recently they had a lude mask costume which increases effect of healing by 20% but the effect expires in Dec 2013.
Very rarely they give out costumes with permanent stats but these often come from once a year events.
Costumes are widely used across other P servers as well and are a delight among the hat collector community. Now they can play with their favorite Western Grace hat look while actually wearing an ugly AoA.
This opens a whole new chapter and potential for revenues as well. There could be free costumes as well as well as occasional AD shop ones.