There's been quite a bit of talk in my bee group about the importance of warm hives. There are some reasonable arguments. I haven't found an academic article to back up this view yet, but that may come in time.
The honey bee rears brood at 35C-36C. When the hive is cooler than that the nurse bees will warm the brood with their abdomens. This thermo-regulation assists the developing brood, presumably by enabling enzymatic reactions. Varroa are said to favour cooler brood conditions, but I have no evidence to offer for this.