I think it's because the admins can't moderate every possible combination of things out there. For example if User A makes a skincent that includes half of a penis, and then User B makes the complementing side, then if you combine A and B's accents you get a large penis on your dragon.

Not saying that it'll happen, but it's probably because the admins want to avoid incidents like that.

Not to mention if they allow it with the caveat that all combinations must be approved before application (like the current skin submission process) it'll be its own monster.