I liked a lot of the flowing clothing options other than capes... Bolero dress, Trenchcoats, Gunslinger belt (female), a few other options. You can only really have one graphic item flapping around behind the character, or you see some major clipping issues. So for a lot of characters, that meant that capes were out. For others, backpacks or wings blocked the slot. For some, it just didn't fit (a stone character).
So, basically, a lot of characters really didn't have a realistic option to add a cape to the costume. From those that did, I would usually add it... more costume items, as long as there is an overriding design goal, usually allows for a better-looking design. As long as the design goal is not simplicity, of course.