I no longer do hard sparring, as a result of concussions (mentioned it before, they mostly came from hard sparring, with and without headgear). I actually don't practice striking in general because of risk of future concussions. But I have done hard contact to the body, with no contact to the head as well, and that worked fine, it was actually my personal favorite. It tended to result in lower guards, but still good fun. And I've done light/medium contact without gloves/protective gear. Honestly, I didn't notice a huge difference in my skill brought about from hard contact and light/medium contact, or any changes in my tactics. The only big thing is that doing hard contact gives you a bit more respect for the other persons strikes. I think that if most people trained light to medium contact (with or without gloves), and hard contact on occasion to the body (with or without gloves), they would get a lot of applicable use from their training. I don't think there should ever be hard contact to the head, with or without gloves and headgear, because I've experienced it plenty of times and honestly getting hit more in the head didn't seem to make me any better at absorbing hits to the head.I'm going to go out on a limb here and assume you don't do a lot of hard sparring. The difference between getting hit with a 16 oz glove and a bare fist aren't even the same universe.
Unless you are talking about heavilly regulated no headshot sparring or stop after one point style sparring, in which case going no gear is pretty common.
To clarify I'm talking about full continuous sparring with striking and grappling, or full continuous striking only sparring.