Ours have gotten rain rot while wearing blankets due to high humidity. I've had healthy horses get more rain rot while IN A BARN during hurricane season both with & w/o blankets. NC humidity causes condensation in barns creating lots of problems (and for some reason the barns here seem to all have solid walls, it blew me away cause i was used to each individual stall opening into either runs, paddock, alleys or large pastures) from June thru Sept (warm to miserably hot here). Just the heavy dew here during the early spring and late fall can cause rain rot. Seems to depend on the year. I owned an arab mare for 10 years. She was in great shape. Never had rain rot issues - even when she stood on her own outside in the worst weather. Then the year many of ours got rain rot, she did too - the only time she ever did. I spent more time that spring/summer doing fungicide bathing than anything else. We didn't have a lot of rain that summer but the humidity was terrible. If I remember right, we also had a lot of hoof problems that year (all ties into metabolism), too.
Personally, blanketing is like feeding. It's individual to the horse, the environment and what you are doing with that horse. Most times ours don't get blanketed, but I do have blankets (some are water resistant, some not), most aren't waterproof however, so would be a problem if really wet out there (which NC generally is). If a horse is healthy he is usually better off w/ being able to use his natural insulation of a fluffy coat w/ regular oils in it. Rolling in that mud will also (sometimes, you do have to watch) help - others it will cause the hair to pack down and become a problem. Show horses who have had their coats removed for showing, older horses or horses with metabolism problems (doesn't just mean old) may need not just one blanket, but several. THEN, someone also has to be around to remove those layers as the day warms up or to put more on thru the night if the temps really get bad... A horse who has his coat pressed flat by a blanket loses his natural fluff and insulation and if he then sweats and the temps drop - you will have one miserable, possibly very sick horse.
And NC does get snow - even here just a couple of hours from the coast - in the sand hills region.
AJ did wear a blanket. This was originally one of his daughter's blankets (larger than he is) and it's big, but better than nothing. He was 18 1/2 yrs old and first time w/ some issues. He is happier outside though. He had the blanket off during the day while the sun was shining, and put back on when the temps started dropping. With the prolonged cold, I used a sheet under it as well. He passed at 20 yrs of age. The 2nd pic is the same storm - the two pastures were back up under the trees (outside the fences) and the horses and the ponies stayed pretty dry and happy. They had access to the barn - which was open - and neither of these groups came in (normally they did not have access to the barn - gates were opened for access for the duration of these storms which strung out to about 2 weeks).
The biggest problem here is the fact that it often warms up after the snow lands and then refreezes into a solid sheet of ice. Doesn't happen every year, it had been 5 years since we'd had a previous storm similar to this one (more snow). We go every 5 yrs or so and get really heavy snow and ice. The year this one happened (2010/2011) - there were a lot of horses euthanized due to sliding on the ice (badly damaging tendons or breaking legs) or worse breaking thru the ice on a pond in our "neighborhood" (west end of our county). I sometimes bucketed water OUT to the ponies where they were in the pasture so that they weren't walking on sheet ice. Ours all did fine. NC literally closes down when the weather does this - pipes freeze/burst, power lines snap due to weight of ice or trees down on them, roads are "funny" (I think they are banked wrong) and so many people don't know how to deal with it - driving too fast and "skate" right off the roads. I was happy to have a job then that I could stay home when the weather was like this!
This was a problem for me after it froze back up. I fell more than once that winter - I tore muscles that took forever to heal.
I had one filly that I'd been keeping in the barn and led up to the pasture where those boot tracks are. She ended up getting switched to a different pasture for the duration until a full melt came. She was so light her hooves wouldn't even scratch the surface of that ice. After this pic, I took a tamper and broke the ice up where I walked, helped a little... When the melt came it was still really slick. That was the filly that was described in another post that we had a time with fitting blankets to. For a liner under the blanket, I used one of my own sweatshirts and put a tie in it to draw it up snug to her belly.