Does it make sense to have a spare derailleur hanger for replacement? Practice shows that the answer is yes. Sure, you do not wish to be left without your bike on a beautiful summer day!
You take a spare tube with you when you go a long way, don’t you? Most likely, yes, because you know that wheels get punctured much more often than we would like it. It’s the same with the replacement derailleur hanger: the likelihood of an unpleasant accident that will break the element is rather high. An accidental impact, a branch or a rock in the transmission, busted adjustment of the derailleur, shifting speeds incorrectly under a considerable load – all this may be a fatal and, alas, the last accident for your derailleur hanger.
If you have another replacement derailleur hanger for such cases, you will be able to continue your ride after not more than 15 minutes. But if you had not taken care of it in advance, especially if you have, unfortunately, managed to ride far away from a bicycle store, a fascinating journey back on foot, in the company of the broken bike is guaranteed for you.