I parked my bike at the Davis Square subway station a week or two ago, hanging my helmet on the bars as usual. A woman waiting for the bus asked "aren't you afraid someone's going to steal your helmet?" and I explained that no, it was cheap enough to be easily replaceable ($30 new) and not worth stealing (resale value $0, since it was old and battered and Boston has a group that gives away free new helmets, no questions asked). I used fewer words than this, because I am a laconic motherfucker what is never verbose.
After a little back and forth, I said "if somebody needs my helmet that badly, they're welcome to it" and left to get on the train. She said "ooookay" in a voice that said
"(1) I think you are crazy; and
(2) I am going to steal your helmet as soon as you're gone, just to make a point".
When I came back 12 hours later,
What do you do with your helmet?
(NB: normal bicycle helmets. Motorcycle helmets and special nice gear you only wear sometimes are a different matter.)
Also, my lights are way more resell-able, and I leave those on, too. Thieves take note: they are coated in ball python venom.