I actually had a good conversation with an acoustic player about this once. There was a lot of good points and some science to back up his point that gig bags are better than hard cases. First, the psychological aspect of handling a guitar in a bag verses a hard case, but also the science behind it also.
A lot of us feel safe about our instruments in hard cases, and it makes us take risks that we wouldn't make with gig bags. I feel like I could throw my HSC down a flight of stairs, but science and statistics show my instruments would be just as likely, if not more likely to be damaged in hard cases than in gig bags. Most often, it's whiplash that snaps a headstock (already under tension from the strings) right off. Hard cases don't do much to stop the energy from an impact from going right into the instrument. A case could fall over from a standing position and create enough force to snap the headstock over. That's why more guitars snap in hard cases than in gig bags.
Now, some science. Gig bags are better for the same reason modern vehicles crumple like beer cans when they get in minor fender benders (Don't text and drive!). Padding absorbs the impact to prevent the energy from being transferred into the instrument. Of course, not all cases are created equal. Custom flight cases with form fit carved foam inserts will absorb impacts, where a cheap hard case with little more than wood and carpet has no give. Likewise, canvas bags might as well be garbage bags, where modern gig bags are like form fit pillows. The padding protects your guitar better than the shell. Best case has both.
On that note, some cases should be treated like bicycle helmets. If there is never an impact, it will last forever. Once the foam insert begins to deform, where strap buttons go into the foam, they should be replaced. If used properly, they only protect you for one crash. That's why my Spector cases need to be replaced.
