Have used them often on bottle babies. Never used them when the calf was with the cow although if they were in a stall or very small lot I think it could help if the calf is small, or weak or has an issue. When a calf is with it's mother, it will get up and suck frequently which will help to keep the gut tract working which generates heat in the body. Plus the cow licking the calf. But with babies that are housed in a calf hutch, or barn/stall or whatever, and only fed 2x a day they are very beneficial for the first few days/weeks. Especially with small breeds like jerseys, or weak calves. It contains the body heat so the milk that the calf is drinking will go towards some growing and not just to provide fuel for the calf to generate their body heat. The other thing, if a calf has a DRY area out of the wind, to bed down in, it can take alot of cold. They cannot take cold wind or wet conditions.
I just returned a couple to a dairy farmer that sent 2 calves home with me late last spring during a real windy spell. I repaired a place that had been torn before I used it, sewed a patch on it and it was ready to wash. They do get soiled, but most all I have seen are completely washable.
Would keep one or two on hand just in case for all the calves I start on the nurse cows and the few that are on bottles. A dry sheltered spot out of the wind is worth more than a calf blanket.