"So before I pull the trigger tonight and order a blizzard bag, any compelling reasons that I should go with a bothy instead?"
Since this is BPL after all: weight. A bothy is much lighter than a blizzard bag. Even solo, a 2 person bothy is 8oz. So for the weight of the blizzard bag, you also get to throw in a down vest, or bump up from a light down coat to a heavy one. But are you always alone? Does a skiing group of 4 really need to carry 3 to 4 lbs of blizzard bags? The bothy can be shared in a group and works out to be 4 oz per person.
Also, the bothy lets you avoid the emergency. You can take shelter and tend to someone injured or hypothermic, repair gear, change layers, navigate out of the wind, or just eat and rest for an hour or two.