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.