In Canada use of oxygen required if between 10 and 13 thousand for more than 30 minutes, or any time over 13000. This could either be conservative, or way too lax, depending upon the health of the individual concerned. I argree completely, that as a regulatory "middleground", it is entirely appropriate. I've used the "Mountain High" oxygen during flight testing, and was very impressed. With the good equipment available today, there really is not excuse for not being safe.
I've had dopey (okay much more than usual for them) passengers at 8000' in the 310, and I crossed part of the Atlantic as the third pilot in Twin Otter at 15000 feet for 8 hours, with only the occasional breath of oxygen, if I wanted to venture to the cockpit to get it. Up there, if you do not exert yourself, you might be fine (certainly health and youth dependant), but any exertion, and you will probably pass out. Summer before last, several of us crossed the Atlantic in a DC-3 as passengers, under a doctor's direct supervision at 16000' with only occasional use of oxygen. We all did fine, including the doctor (he was 70 years old, and I suspect healthier than any of the rest of us!). We were all watching each other very carefully. The extreme for me without oxygen was a quick up to 17000' and down, in a Cessna 180 amphibian (neither I, or air traffic control thought it would go that high!) I was young, healthy, and dumb, and it was not a good idea. I would not repeat it without oxygen.
I do remind myself that their are people who live at 13000' and more, I bet they're healthier than I am!
Pilot DAR