I toured across North America without a lock. I had as much money tied up in things hanging off of the bike as the bike itself with no practical way to secure everything. If I lost my panniers and contents, tour was over, bike or no bike.
My gut feeling (or hope anyway) is that the opportunistic thief (hey, there's a bike, let's steal it) will abandon the plan when they grab it and feel the weight of the whole mess.
I've also been know to ask people on the street, shop vendors, etc. to keep an eye on it for a few minutes if I didn't like the security situation.