When my wife was my girlfriend, she bought me a very expensive new helmet that I wanted. We rode to the beach a few days later and locked the helmets to the bike. Came back and found somebody knifed the strap, leaving me just the D-rings, and it's not that hard to sew on a new set. A friend and I were out on another night and my riding buddy found that somebody dumped their soda in his bottom-up helmet.
The only way I've locked the helmets since is in a saddle/tail bag, or with a cable where I look through.
Bob