Trace, you make a number of valid points. At the end of the day, if we are truly diving alone, and don't especially care if we are equipped to assist someone else who might be diving near us, then there is really no "right" way to configure gear.
In my case however, where freeze up is a constant "threat", then having a completely redundant, isolatable (is that a word?) system is mandatory. I learned this the hard way a few decades when my primary reg froze up and I ended up with a massive free-flow at 170' on a single 72. I rode that tank up as far as I could before switching over to a 13 cf pony. By then I was up to about 30' and was able to hang for a bit. That was about the last time I dove a single tank up here.
I was fortunate that this happened early in the dive so I hadn't racked up much deco obligation, but it would truly suck to suffer a major free flow when you have a mandatory hang and nothing to do it on!
One of the tricks with freeze-up is that the most likely cause is having a little moisture in the gas. Since most of us fill doubles both tanks at a time, both tanks will have the same gas in them. If one reg freezes, then there is a reasonable chance that the other one could as well. (There was an unfortunate case of an experienced diver dieing up here a few years ago. He was under the ice and had both regs, plus both inflators freeze pinning him to the underside of the ice. He had a buddy, but he was unsuccessful in helping...)
If you are an "aware" diver, you can generally sense when a second stage is "thinking" about freezing... a little extra flow after you stop inhaling, for example. When I feel that, I'll switch regs to give the first one a "rest". The water is above freezing temp, so it will thaw any ice forming around the second stage valve...
I'm going to stick to my double configuration, but that is dictated largely by the cold water here, and the depth I generally dive.
