Brain weight probably has more to do with backcountry safety than equipment weight.
I carry more first aid gear gear than most ultra-light gear lists show, but the philosophy of less-is-more certainly helped me cut down there as everywhere. It is the main area that really struck me as a safety issue with UL gear lists-- I really want more back-up than a few bandaids and some moleskin.
I agree that lighter loads are safer for balance, falls, stress on joints, overexertion, dehydration and heat-related issues.
Many in this thread have concerns with durability. New materials breed new techniques. This has come up in many areas: bicycle, motorcycle, automobile and aircraft design all come to mind. Again, the less-is-more concept makes for lighter gear, first of all be leaving off all the extraneous fittings, pockets, zippers, etc. Next the load in the gear is lighter for using the same less-is-more concept and the lighter materails used-- we get a nice snowball effect right away. THe lighter pack load allows use of lighter materials and the snowball continues to grow. Thin fabrics require a little more attention, but I was never a fan of throwing my gear around or sitting on my pack as one person told of here. I've seen some sleeping bags ripped now and then, but is was more inexperience and accident than material durability.
The other area of the safety of UL gear centers around appropriate gear for the conditions-- going beack to brain weight rather than gear weight. Knowing how to pitch a tarp takes a little knowledge and practice. Tarps make sense to me in summer, but offer less protection in windy wet weather in fall/winter/spring travel. I live in wet country and prefer a little more shelter than a tarp, but going back to the less-is-more idea, I don't need a 10 pound tent to keep myself dry either.
Another related question to ask: is UL travel healthier? I think so. It is less stressful on body parts, and offers better protection from heat-related issues. The effect on attitude and enjoyment of the journey goes without saying.