Great points, Steadman. In fact, I do carry a pair of nitrile gloves; the only catch is that they might not be sterile if I go to use them, as for me they're primarily a vapor barrier glove. See this BPL forum thread for some thoughts on that if you're interested (not directly first-aid related ...).

But I'm carrying them anyway; they'll protect ME while I'm helping someone, and of course some sterilization can be attempted with purell or whatever.

Cutting up clothing: good point, of course, and particulaly for the light-to-ultralight backpacker who is already weight conscious --- likely they have less spare clothes. In the Army they told us to always use the other guy's first aid kit, so one possibility is that the victim has spare clothes ...
but at the end of the day it could require some careful trade-offs. I figure I could cut up a silnylon stuff sack and leave some stuff loose in my pack as a result (for example), or cut a strip or two off the end of my poncho without leaving me overly exposed to natures tender mercies.

It's so hard to have the "right" first aid kit in that you can't really key off of what other people bring so much. I.e., if I watch what experienced folks use for sleeping bag, tent, etc, that's going to have some experience behind it --- because you use those things on the trail every day. Thru-hiking the PCT in 2008 I went for 2600 plus miles without ever using anything from my first aid kit in terms of actual injury --- I used a little body glide, some duct tape on blisters early on, and of course a little ibuprofin now and again but that was it. So my "experience" at what a person needs in a first aid kit is pretty low despite having a lot of trail miles. My understanding from others is that what first aid kits often lack is enough sterile material to soak up blood and stop the bleeding (hence the female sanitary napkin(s) idea).

Really, it seems to me that "first aid" stuff can be divided into the two categories of "stuff that tends to get used" and "stuff I hope I never use". The former category includes things like moleskin, anti-chafing, anti-monkey butt, pain medication of choice, that sort of thing. It's ultimately the latter that I think is hard to plan for, balancing dead weight carried against dealing with unlikely but potentially severe situations.