From what I've read, the major purpose of locking your bike at controls on PBP is so that it doesn't get moved when you're not looking, so you don't waste time trying to find it.
As a general point, even if you are locking the bike, it's with having some way of easily identifying your one among the hundreds that will be stacked up at the controls, especially at night. It may have looked a bit daft but I have always had a small, brightly coloured cuddly toy fixed to the handlebars or saddlebag for PBP or LEL. It makes it much easier to spot the bike among a sea of identical-looking handlebars and saddles.
Echoing other comments, I always take a lightweight lock too, for cafes, shops, etc. Of course it won't deter the serious thief - only carrying half a ton of metal would - but it should put off any casual opportunists and I wouldn't want to risk a long, miserable walk to the next control / railway station.
