We don't blanket unless it's some sort of special circumstance. Horses will grow a winter coat as thick as they need it. I've been told that blanketing (like many other things) is more of a comfort to the owner than the horse.
I have actually heard that blanketing in cold weather is more detrimental. Something about the hair not puffing up/out and insulating them..? At any rate, I don't know how true that is, honestly.
As far as rain goes, unless you have some sort of plastic rain sheet, I would not put a blanket on them. Standing out in the rain in a blanket means that in a few minutes, they are going to be wearing a sopping wet rag that will do nothing whatsoever to protect them from the wet.
If you're really worried, you could bring them in the barn if you have the option. As long as they have a shelter and the option to get themselves out of the wet and cold, I wouldn't worry too much if they decide not to use it. A lot of them don't.