Lupin fights back in kind, undermining the power Snape has over his own students, and does it in such a way that Snape has no real recourse.

Yeah, Lupin is almost Slytherin there. The one problem with this technique is that it will only work as long as Snape is willing to avoid a scene.

Someday Lupin is going to goad Snape with one of his "you can't touch me!" remarks, and Snape is going to decide that choking the life out of Lupin is *worth* the trouble he'll get into -- we have seen Snape totally lose it once at the end of PoA, and also outright defy Dumbledore's wishes in OotP over the Occlumency lessons.