Small U-locks are not very practical for locking a loaded touring bike to a solid object because often the panniers get in the way. You just can't get the bike close enough to use the U-lock. A good quality cable lock is the way to go.
Very good point.
On my upcoming tour, my buddy and I will both be carrying cable locks. The plan is to lock both bikes together with both cables then lock to something solid.
I was also considering taking a ulock along, just to lock both bikes together. The only city I am really worried is San Fransisco and I will only be there for 1.5 days. On the whole tour, I don't foresee leaving the bikes out of sight except when I might explore Hearst Castle. Is the ulock overkill?