I think it's possible that the widespread use of non-lethal weapons could result in greater attrocities than the use of lethal weapons on the macro-level. It may not always be aparent, but in general, there is human restraint against killing other humans. A group of people would have to do, or at least be pegged with doing, something fairly high on the "bad scale" to justify use of lethal force. I suspect the threshold for beating them into submission with shotgun fired beanbags, high-pressured water, etc. would be much lower. At the level of an individual life, non-lethal would obviously be better. But at the level of one group of people trying to exert their will over another, I think non-lethal weapons would lead to more of that exertion, because it would be less difficult to justify.

When an insurgent points a RPG at my Humvee I should fire [far less lethal] rubber bullets back because I have better air support?

I can't find a reference now, but I've read in the past that the 5.56mm round -- the ammunition used in M-16s, for example -- is designed with a goal of (among other things) seriously wounding the target rather than killing. This isn't for humane purposes, but because putting a soldier into the infirmary with medical staff necessary to support him poses a greater cost to the enemy than simply killing him.

Most "non-lethal" weapons are short ranged. You can't fire a nonlethal bullet from the air.