People frequently use the face mirror to be able to tell what facial expressions they're making with their avatar. Additionally, people often have touchable physbones on their heads, including hair, ears, etc, so the face mirror allows them to see when they're being interacted with.

However, in order to see when these things happen, you have to have the face mirror toggled on all the time, which can be distracting!

Allowing avatar contacts and expressions to toggle the face mirror would allow for things like "show the face mirror only when hair/ear physbones are being touched", or "show only when making certain expressions". These behaviors would give the user feedback about how they're being interacted with/interacting with others, without having to look at your own face 24/7 or go into the menu to toggle the mirror on and off on a regular basis.