I have, perhaps, a little different prospective on the subject. This may be partially due to where I live, Texas. Over the years, I have seen as Tinboatcapt pointed out, instances of helmets stolen by cutting the strap, but that was primarily years ago. I have seen helmets blown off their perch on the bike seat and hit the ground. Throw into the mix that in 29 years of Police work, I never saw a report of a helmet being stolen off a bike. For me an added consideration is that when using a helmet lock on a bike, whatever grease, road grime, etc. on the bike can transfer onto the helmet. Then too, many of the owners manuals for various helmets caution against even putting them on a mirror like we used to do back in the day. I think actually, a tank bag and contents, and/or a bag and contents strapped onto a tail rack, would be far more attractive to a potential thief than a helmet.
The sun in Texas is brutal for many months out of the year, which is not good for the helmet materials longevity. Then too, it is not pleasant to put on a hot helmet that has been baking in the sun for any length of time. Up to recently, all my helmets had D rings, but my new Shoei Neotec II has a stainless micro ratchet ( I like it a lot ). Whether D ring or micro ratchet, I always fasten the strap after getting off my bike, use the strap as a carry handle, and carry my helmet with me wherever I am going off the bike.
Personally, what I would LOVE to see, would be bike manufacturers changing the helmet lock on the frame to a factory universal trickle/tender battery charger plug in point, with a waterproof rubber cover. I am certainly not disparaging those that like the helmet lock, it is just a matter of what works best for me.