Not sure why a medium weight would not work for a horse with a coat, but… I do have much to learn.
This is one of the biggest misunderstandings when it comes to blanketing.
For a human, if you are a little bit cold, you just put on a little jacket. So they think if a horse is just a little bit cold, that they only need a little bit of a blanket. WRONG.
You have to understand how a horse keeps themselves warm. When the temp is around 40 to 50*F, the horse is neutral. They don't have to exert extra energy to stay warm, or to cool themselves. When the temp drops lower then that, they "puff" up their hair coat to create air pockets around their body. The colder it gets, the puffier they will look.
As long as they are in good health, are DRY, and have access to windbreak, most horses are just fine in any temperature.
I live in North Dakota. Just last week, the "feels like" temperature with the air temp and wind chill was -50 below zero. Yesterday, the warmest it got all day was 1*F. I don't even own a blanket. But my horses are healthy and they have access to food/water 24/7 and access to windbreak. They live outside all the time. They are fine.
Anyway, getting back to the fact that you are putting a medium weight blanket on your horse ...... When you put that blanket on their back, you are SQUASHING down all their hair. They can no longer "puff up" their hair to make air pockets. With below zero temps, I wouldn't think that medium weight blanket would be enough. And now you've just done more harm
than good, because you've taken away his natural ability to regulate his own body temp by squashing all his hair down with the wrong weight blanket.
I also see horse owners who have WET and shivering cold horses and what do they do? Throw a blanket on without drying the horse off first.
Again, does more harm
than good if you don't dry your horse off before blanketing them.
And another point that horse owners often get wrong. They think that a horse needs to be warm and TOASTY under their blanket. WRONG. If you put your hand under your horse's blanket, it should be neither warm nor cold. It should be neutral. If it is too warm, your horse will start sweating, and then we have the same issue of a WET horse. What happens if you get sweaty under your clothes while out in the cold? You start to get chilled. Same with the horse.
I have no problem if a person chooses to blanket their unclipped horse. But do your research FIRST so you don't end up inadvertently harming your horse.