I believe they're simply little rewards for owning the other games. Granted, I agree that these should be costume PACKS that come with an article of clothing for each selection (Headgear, Headphones, Facewear, etc) that come default in the costume's colors. This way you have the costume, but you can still mix and match different pieces with the rest of the game's clothes and customize their colors.
If they really want players to spend their money, this would be the way to do it.