Lots in play here in the probability of theft (where, how long, appeal of bike, etc.), compounded with personal aversion to risk, and one's own personal load weight / perceived value balance. The "rational" responses range from keeping-the-bike-next-to-you-at-all-times to completely abandoning it compounded with U-Lock / chain to no locking system. Once someone has their bike stolen, one generally moves more towards the proximity / lock quadrant.

I carry a lightish cable lock, bring the bike indoors when possible, but also keep a furtive eye on it. Campsites are slected bearing in mind not only a level dry place to sleep, but also whether there is something solid to lock the bike to. My locked bike is always right in front of the door of my tent.

When my bike is stolen, it won't be because (in my own mind) I didn't take reasonable precautions against theft, it'll be because it was my turn.