Bear in mind you only START with 6 costume slots. Which means you have to do at least one of the costume missions to have enough slots to have a single costume for yourself, and 6 slots reserved for your pets. So yeah, for a lot of people they wouldn't want to burn almost all their costume slots just to modify the look of their pets. I generally have 5-6 different looks for most of my characters. Costume, casual clothes, an alternate costume, dressing up nicely, etc. So sacrificing nearly all my costume slots for pets would be frustrating, and I'd much prefer a better way to do it.
