The "Show Cage" function doesn't work on the modifier stack base-level Editable Poly. You have to add an Edit Poly modifier to the stack to get it to work. Seems like if there is a check box to enable it, it should work. I use this feature all the time during retopology, as it allows me to see the wireframe of the mesh I'm creating without having to also see the distracting wireframe of the much more dense mesh I'm referencing.