It struck me, when I read it this time, that this was a book about an out of control arms race. It was as if our own Cold War suddenly got Hot.
The wizards knew (it seems) that their actions could cause the end of the world, and yet they just couldn't stop themselves from hurling spells at each other.
The role Rincewind plays is most significant in that he realises that someone has to put a stop to the madness, and he accepts that has to be him. As Sister Jennifer says - this is Rincewind being a real hero.
For those of us who lived through the fear of the Cold War, this is an interesting parallel.
“Men never commit evil so fully and joyfully as when they do it for religious convictions.” – Blaise Pascal