Unfortunately, I've heard plenty of stories of bikes and their parts being stolen. Whole bikes lifted up and off of the sign posts they were locked to. Cable locks quickly snipped. More than a handful "Well... I actually forgot to lock it..." Hundreds of wheels, and seats/seat posts taken by (best case) hungry orphans who needed the bike parts more than you.
Yesterday, a woman dragged her bike in because a hungry orphan stole the front tube from her bike. Just the front tube. The thief went to the trouble to remove the wheel from the bike, remove the tire from the rim (with the tire levers he/she just happened to have in his/her pocket), remove the tube from the tire, and then leave the wheel and tire in a nice pile beside the bike.
That's the first time I've ever heard of someone stealing exactly and only the part that he/she needed.
If you're ever in desperate need of a tube and you're thinking about stealing one- don't. Just stop by the shop- I'll give you one and we can work out a payment plan.