If the Serpentine Belt Breaks, What Exactly Happens?

Part of the "use old belt for spare" vs "buy a new one and let it age out" depends on how difficult it is to install the belt.

On our side radiator coach, belt replacement takes 3-4 minutes. But, on some rear radiator coaches, it takes two people and considerably longer.

This SHOULD factor into the equation. If labor (yours or pay someone else) is easy, stick with the used one. If labor is very time consuming/expensive, carry a new one.

I have no problem with either answer.