This is a very debatable subject and every one who has an opinion is very sure they are right.
I think there are a lot of variables to consider. Regardless blanketing will reduce your horses ability to respond to cold appropriately.
Rain is tough to deal with in my opinion. I would think the effect of continuous rain would be most adverse to hoof health and condition.
Horses do tend to cope fine with rain & cold. I think rugging is *very commonly* unnecessary & overdone. But of course, if that means inundation for long periods, if the horse is ill, old, without shelter, etc, etc, then rugging may be best option. Whether that's a 'rain sheet' - I presume you mean a very light, waterproof 'rug'? Or whether you need to consider a thick rug for warmth.
It is very common for people to over-rug, even to the point that it may affect health. Many people rug constantly without respite, even for keeping (show) horses clean, more protected, coats short, sun off, etc. People also might put rugs on a horse at the start of winter & rarely take them off till spring! Rugs, particularly if heavy or constant, can obviously cause the horse not to be able to regulate his own temperature. Even in absence of a rug, as they may a)acclimatise and b) muscles under the skin that control the coat may atrophy. So you also need to consider whether you can get to the horse to remove/change the rug when it's warm. They can also make fungal infections more likely/major & other effects. Therefore it's important to think about why you're rugging & the possible negative effects too, when deciding how, when & why to rug.
If you're smart & considerate about it, I don't believe it's a problem.
Thank you both very much!
T opened the thread in the wrong forum and only now noticed,
Lol sorry about that....
In our country most times it's very hot and I doubt that we will need
To cover our horse, though some of our winters can be very harsh
In a very unexpected way... I think we going to just let her deal with it naturaly,
She does have a shelter and she's not old/ill
Thank you for the helpful comments :)