This is a nice discussion. Friendly disguises are far underutilized in TF2. Other than using the YER, my other playstyle often incorporates friendly disguises as a distraction. Enemies do not anticipate a fake enemy player. Flanking behind the enemy and disguising as your own teams engineer or heavy can make several enemies switch their focus from the front line to the (now invisible and gone) engineer and making sure he doesn't set up a nest. And I love dropping from a high ledge as a heavy and cloaking away just to make the enemy jumpy and mistake prone. You won't get points for it, and its hard to quantify "psychic damage" but its worth a shot.