Originally Posted by jz19
I have read that replaceable hangers are there to protect the derailleur in case of a crash not the frame. The logic been that it is cheaper to replace the hanger that the rear derailleur.
Actually the replaceable hanger is intended to protect the frame, not the rear derailleur.
There are intentionally weakened derailleur mounting bolts that are supposed to break before the derailleur is damaged but I don't know if they work and haven't seen them offered for some time.