Not to say the costumes are perfect, either, but they're cheap to buy, free to apply (and dye) and are normally better looking than at least 95% of the armour.
Taesci for the Shoulders (light in both motifs are basically arm bands, yay!)
Sure, they could add all slots. And some people would have maxed slots, account wide... and they never would buy another thus cutting their profits.If people paid for them on multiple characters, either refund them or increase the total of account wide slots available.