Except you do. Having slots means the game needs to keep track of what's in every slot. Vanity and non Vanity. The way they're doing it, instead of the body piece you're wearing pointing to visual A, it points to visual B instead. With secondary slots, it has to keep track of those slots as well. Even if it's only a small amount of memory used to do so, when you're breaking yourself over 256MB as it is, anything helps.