Originally Posted by dddd

Many times when the derailer is traumatized, the two main (mounting and cage) pivots are no longer parallel, irrespective of any cage replacement.
In these cases, the derailer's indexing will be off as the cage would rotate off-axis relative to the axle and sprockets, thus making proper cable adjustment impossible, and also being affected by shifts at the front derailer since the cage rotates a lot whenever a front shift is made.