I agree that the best solution is not to run out of air in the first place, but you are misapplying the rule of thirds. It's not intended for basic open water diving. That would mean on a 30ft dive when an AL80 would last you an hour, you're limiting your dive to 20 minutes. Way too conservative and not very practical.
My arithmetic indicates that 1/3 out, 1/3 in and 1/3 in reserve provides a 40 minute dive, not a 20 minute dive. If one does not count the return trip as part of the dive, then neither should the trip out be counted. Following that logic, the dive time in the example cited would be about zero.
Most of my dives are not that linear, not a simple 'out/in' plan. Typically, I dive over or enter very near to a defined area where I will be fish watching and exploring. I consequently apply the rule of thirds to planned activities and the area involved, calculating a maximum expenditure of gas at 2/3 of the quantity available. This requires a comprehensive pre-dive plan, and contingency plans should circumstances change.
I usually dive on jetties and inlets, generally not deeper than 35 fsw, and frequently at night. I carry a spare air on these shallow solo dives, though I have not ever had to use it in the 30+ years I've owned it. Like the multiple (4) cutting instruments I carry in these fishing line infested areas, it's a small unobtrusive safety measure. I try to keep a sleek streamlined profile while diving. The knives, shears and cutting devices do get some use.
On deeper dives I use a 6cf pony, and also have never had to use it except to check that all is working properly. 6CF is adequate for me, since I imposed an 80 fsw depth limit on myself when I reached the age of 70. Perhaps I'm too careful, but I've been diving a long, long time, and I'm still here.