A lot of great advice here, and it does depend on other factors, such as, are they stalled at night, do they have a run-in shelter, do they grow good winter coats, etc.
I think in general it is best not to blanket as long as the horses are in good health and grow nice fluffy winter coats.
However, for older horses, I also don't think it would be a bad idea to get a couple of inexpensive blankets, perhaps for those nights when it gets really extremely frigid, and for certain weather conditions. Hard driving, cold rain or sleet can soak their coats and leave them very chilly, so giving them a layer of protection against that kind of yucky precipitation is a good idea.
I usually don't advocate buying 'cheap' blankets, because you really do get what you pay for, and it's a better investment to buy a quality blanket if they are going to be used daily.
But for only occassional wear.... there are many very inexpensive blankets out there. I've been in the same situation, where I had an older horse that lived outside (with a run in barn) and had a blanket only for occassional use, like freezing rain or nights when the temperature dipped way below normal. So I had a very cheap blanket, only used it a couple of times a year and it held up great for many years.
Take a look at Dover Saddlery, I think they have the best selection of inexpensive blankets. I would look for something cheap and waterproof.
Or perhaps something like a waterproof sheet (just to help the horse stay dry during freezing rain/sleet) and if you feel like it, try making your own blanket liner that can go under the waterproof sheet on very very cold nights. I made one out of a thick fluffy polarfleece, bought for only a couple of bucks at the craft store, held in place securely by a web blanket surcingle (can be bought at Dover for a few dollars.)
It was great, because I only used it a couple of times a year under the cheapo blanket, and it cost me a fraction of what a premade blanket liner would cost from the catalog.