This was the standard for many Canadian Commercial Beekeepers for many years. They chose to kill the bees and start fresh each Spring 1) to salvage as much honey as possible in their cold climate 2) reduce the cost of chemical or natural treatment of bees to reduce Varroa and other pests and 3) to reduce the colony loss due to Varroa mites - nothing is worse than using up a great deal of your honey just to lose your colonies to Varroa.
There has a been a paradgym shift in thinking as the cost and availability of replacement bees has sky-rocketted to the commercial industry. Now they find it more cost effective to hive the bees in buildings that are warmed enough to reduce the consumption of honey and even commercial beekeeper "associations" are banning together to buy chemical treatments in high volume.
It is a war out there - the beekeepers against the Varroa and it appears that SLOWLY Varroa resistant bees are making their way into the feral hives. Nature itself sees that this is a critical situation of survival and is making changes (although it is unclear what changes beyond possible small cell building) but something is happening - but NO WHERE NEAR THE RATE that Varroa is killing off bees around the Globe.
I can not morally justify killing my bees unless infected with Foul-Brood. Otherwise I will do all that I can to over-Winter my bees for the following season and then if necessary requeen the hives.
In my case, last year I bought two packages, installing them http://www.beemaster.com/inst2004.html
and I had no sign of Varroa into the Fall - something I am still VERY surprized about.
I had a third hive which was a swarm that took possession of some frames in a super I had placed in my shed - that hive (which now is in Virginia at Bigrog's home http://www.beemaster.com/bigrog.html
) did have a varroa problem and Rog has done EVERYTHING right in handling the problem and I'm hoping that they will be fine into Spring :)
To keep or kill - I'm a "beekeeper" not a beekiller. I vote to always keep your bees if the colony is healthy and strong enough to survive your particular Winter.