I would like to point out that even though I was painted in this thread as a "purveyor of QAS dogma," by the esteemed (pathological liar) Jeff Bartlett, I actually do not fit this label very well. I tell people that I usually carry one, but I prefer to think of it as a "third ascender" because either upper ascender can be used as QAS as superDerek and others have pointed out. I do a lot of training new folks on SRT, and the only time I suggest that they should have three ascenders is if they have a long rack and have trouble with changeovers. I do not suggest (dogmatically or otherwise) that anyone else should have one. The NCRC no longer uses the term QAS either these days.
I'll admit that (much like Obama) my thinking on this issue has evolved. I was originally trained that you had to have three, and the third was the non-integral QAS that superDerek defined. Like others, I realized that the separate QAS has limited use and now only carry it if I have the long rack or the trip is otherwise not very weight/bulk critical. Hell, I can only really go on trips that aren't that weight/bulk critical anyway, although I have lost 11 pounds since spring when I looked on facebook and said "who is that fatty?" and then realized it was me!
I suppose an interesting question is where did all of this QAS stuff come from anyway? I know some of you out there must know the answer to this. I'm sure there were reasons for it, and it would be interesting to review those reasons. The French didn't start it, so it must have been us. Nowadays, we see most Americans turning away from the idea.
If I have purveyed anything in the past, please remember that ideas and attitudes change over time and forgive me, I've still got my training wheels on. I have learned a lot from the minimal gear crowd, and am well on my way to being fully converted. Just a few more audits on the E-meter and I'll be there.
I hope that I never stop learning and refining ideas, gear, and techniques when it comes to SRT.