For good or ill, I'd honestly say that they can add whatever event based cosmetic items they want and it's no big deal. Does it get irksome and immersion breaking to see someone run by in a bizarrely out of place outfit? Yes, sometimes. Really, though, any time you go out to PVE, you're like as not to encounter people wearing amalgamations of varying sets anyway. Adding things just for fun doesn't make much more of a difference by comparison. Just my two Gil.
