When touring I'd be more upset about the theft of my loaded panniers than the bike itself. Fortunately the panniers and contents aren't very attractive theft targets - although it would be very inconvenient and expensive for me to replace them, they have very little resale value to a thief. But that reduces the value for me in carrying a relatively heavy lock which only protects the bike frame and wheels.

Small, high value items (camera, phone, GPS, other electronics, wallet, etc.) I carry in a quickly detachable handlebar bag or fanny-pack that I take with me, but the panniers generally stay on the bike when I go in a store or restaurant. I use a pretty light cable lock to protect against the casual 'opportunity thief' but depend more on locking it in a location where I can keep an eye on the bike and panniers. I've never had anything stolen while on a bike tour and it's been a few decades now since I've had a bike or components stolen under any circumstances. I do have a U-lock, but it sees little use - only when I'm riding into a major city and/or will be leaving the bike unattended somewhere for an extended period.