I use them on all threaded barrels. As Black Commando said, they are cheap insurance.
I have picked up several for as low as $8.95 each at Joe Bobs out in Hays. (shameless plug for Joe Bobs)
Only 2 of my threaded barrels actually came with thread protectors. One was a Glock 17 barrel and one a Savage
30-06 rifle barrel.