You don't need to carry a lock - you can always beg other riders into sharing their lock
<pompous voice> Well, if everyone thought like that, no one would have a lock to share </pompous voice>
Is this an example of "the tragedy of the commons"?
I take a 1980s era cable lock that is sufficient to deter all thieves apart from those equipped with a stout pair of scissors or perhaps a butter knife. But that just lives on my bike all the time. The bikes are rarely (if ever) left entirely unattended - there's always someone milling around, and in anywhere that might actually have a real risk of theft (Breakfast in Hull I remember) then a bike watcher might be appointed.