Currently when you wear a Custom Avatar the Hairstyle, Face Scale, Clothing and Accessories Tabs all grey out and prevent you from using them.
This goes against allowing people to further personalize their avatars. If they decide to create or purchase a custom avatar they have no customization options other than to wear what that custom avatar is.
Unlocking the Tabs for custom avatars will give the user the choice of trying on things they have purchased on the custom avatar. It may not correctly line up based on how different the custom avatar is to the default body, but it offers users the freedom to express their design choices.
Also this will allow creators the customize the default body textures and upload them as avatars or slightly modify features of the face or body but still allow the user to wear their existing items.