[QUOTE keeping the canister warm just long enough to heat up a batch of water.[/QUOTE]
Now that's the real key to this whole "debate" I think. It all depends what you consider a batch of water. For 2 cups to hydrate a freeze dried meal, or some tea, soup, whatever, yes a canister can do the trick with a little technique.
However, I like to boil up a couple of liters for hot water bottles to preheat my sleeping bag and keep me toasty into the wee hours. That gives me 2 liters treated and ready to go in the morn. Then I preheat my cold boots with them. Added to the liter for dinner and hot choc. that's about 3 liters boiled. I don't feel I could easily do that w/a canister. For some, it will seem like way more than necessary. OK, maybe, but I like to be comfortable at night, and I try to drink every bit to stay well hydrated during the day.
Also consider that if someone suffers genuine hypothermia, I want lots of hot water bottles to warm them. Not likely, but you never know.
I was out 2 weeks ago after the last storm, in the Catskills. It was 2 degF. I don't screw around at that temp. Just me.