This really depends on which city you're in and where in the city you leave your bike+helmet.
I've had the ultra-cheap lights on my bike stolen. I've seen plenty of other bikes stripped of everything except the frame itself.
This is why I always take my helmet (and my lights) with me, lock my bike and the front wheel with a good lock, and chain lock my seat down for good measure. Haven't had anything stolen since -- even in some sketchy areas.
I've had the ultra-cheap lights on my bike stolen. I've seen plenty of other bikes stripped of everything except the frame itself.
This is why I always take my helmet (and my lights) with me, lock my bike and the front wheel with a good lock, and chain lock my seat down for good measure. Haven't had anything stolen since -- even in some sketchy areas.