Originally Posted by
BostonFixed
All of the older frames didn't have replacable derailleur hangers.
On an older steel frame road frame I have, the rear derailleur found it's way into the spokes due to my carelessness, bending the derailleur hanger, but the derailleur was perfectly fine. I did re-align the derailleur hanger/dropout with a 5 pound sledge hammer.
The dropouts on classic steel frames were forged from mild steel to allow them to bend for alignment and repair purposes. Modern frames don't retain this malleability and so disposable hangers are often used to allow easy replacement of the hanger when it is damaged.