My 2 cents:
Unfortunately that's no guarantee nothing will go wrong.
If it's made by man, it can break.
I've had failures with sights that go flying, extractor hooks that break off, magazine catches that don't, springs that go sproing, 'etc.
Having spare parts handy just seems to guarantee that particular part will never actually fail.
A spare gun makes life easier.
And as had been said, it doesn't have to be the exact duplicate of the main gun.
Anything that allows finishing the match will do.
You know how to effectively use all your guns, don't you?