Well, one would think you could wear furs over your armour, right? You're supposed to dress in layers to keep warm anyway. It would make thematic sense not to be running around the frozen north only incased in a cold metal shell. There's a fine line between making it harder to survive in a fun way and creating a bunch of tedious tasks to keep up with.
There are a couple of mods that add overcoats/cloaks (including one that brings Game of Thrones style cloaks that fit well into Skyrim's art style) that Hypothermia and Frostfall are aware of and add to the exposure calculation. The problem with both is that your protection is based on the addition of all of your armor's individual protection rating based on the type of armor. It's a strict linear progression instead of a logarithmic progression so the more armor you're wearing, if
it's the right type, the more protected you are.
If you wear heavy armor, unless it's specifically made for the mod and as such has "fur" in it, you're not going to be as protected from the weather as you would with the weakest fur armor. Which means if you end up going through the far reaches of the north (with the armor option turned on) you have to choose between being able to survive the cold, be protected from the trolls and sabrewolves that are up there, or carry both types of armor and switch between them based on current need. It's certainly realistic, but it's not fun.
Instead, abstract it out so that during normal weather you're "protected" as long as you're wearing any clothing. If you enter freezing water you slow down then start losing health, and during blizzards you slowly lose stamina, slow down and start losing health. They can let cloaks or similar protect you for longer than just wearing armor/clothing in those cases. It serves the purpose to make those serious weather types/wet areas dangerous without being something that adds micromanagement or pushes the player to "game" the system. If anyone knows of a mod that does that I'd be all ears.