Well here's my guess at it.
1 - When most guys don't use the stiff arm (I mean how many times do you see a stiff arm in a game?), defenders probably get used to going for the body. So it's habit.
and
2 - I could just imagine the kind heat a guy would take if he gets stiffed armed, tries to grab their wrist or something only to have the guy pull away his arm and seperate from him as he runs away.
You know. It's kinda like when Greg Wesley escorts the offensive player down the field trying to poke the ball out from behind rather than just tackling him.
