This one's a bit weird, it doesn't see much use most of the time. It's pretty important on some levels - where there are lots of faces toggled off at the start, it's very useful for toggling them back on (although then you have to be careful that your opponent doesn't take over the valuable real estate you've just created). But why would you ever want to turn faces back off?
I can think of a few interesting strategies based around this, but I'm not sure that they're ever worth the effort to pull off.
- Make holes for laser turrets to shoot through.
- Make a border of toggled faces along your territory so that it's slightly harder for enemies to break in; they don't have faces to back them up.
- Weaken enemy territory by charging in and making holes in the surface, so even when they capture it again it's not strong with faces.
I think it's worth leaving in there to be an unusual ability with potential applications by advanced players, even if most of the time it seems like a waste of a slot. Time will tell whether anyone figures out how to put it to good use. (Plus, as I said above, it's quite important on certain levels.)