I always blanket. I ride all year round, and I dislike having a huge furryball to brush out when I need to ride. My horse has thin skin, and he dislikes the cold, very much! Lol Where I board right now, it's about 65* in the barn.. so even if I didn't blanket, he wouldn't grow a coat.. and then for him to be put outside- he would be cold. At my barn, they put the blankets on in the morning, and remove them when they come in from turnout.
That being said, my horse has had blankets on since early September. During the summer he wears a fly sheet, then fall time he goes into his turnout sheet without fill, then we add a lightweight, then we take off the sheet/ lightweight and put on his medium weight, and then like now(middle of winterish) he's in his heavy weight. Springtime comes, and we reverse the cycle. He is pretty much always covered.
That being said, I don't see the point in putting a blanket on if they have already grown a full coat? All that you would be doing is flattening their hair and making them cold..