My two have rugs on.
My gelding actually has a thick waterproof turnout on, and then underneath a very warm stable rug [which will soon be replaced with another thick turnout, which unfortunately isn't waterproof yet]. He gets VERY cold, and in fact I'm sitting here at 12.16pm wrapped up in a blanket, INSIDE. My house is a fairly warm house and if I'm cold enough to be wrapped up in my nice thick winter blanket, it's cold enough to leave the under-rug on. Sunshine out one window, overcast out the other... we'll have rain later, too.
My filly is in a light waterproof turnout and while she gets a little cold in it, better that than too hot. She is from an hour away, inland, so it gets even COLDER where I bought her from, and she was never rugged before I got her.
I have a tendency to rug by default, and my filly will be my show horse in a couple of years' time, so I want to minimize the amount of winter coat she grows... she's chestnut and the red ones go the grossest colour when you clip them. My gelding drops weight when he's cold, and he gets cold really easy - a "comfortable" temperature under his rugs is too cold for him. You want to put your hand under his rugs and feel like it's a bit warm under there. This time last year he literally had FIVE rugs on, all layered up. Unfortunately most of those have been shredded since, because they've all been used on Magic, who isn't kind to rugs. She seems to be ok with a standard neck, but combos, no way - they last a few days if that. I haven't tried a rug and neck rug on her.