I have little dogs, both chihuahuas. They definitely need sweaters. Usually at this time of year, they run out, do their thing, and run right back in...but if I am taking them outside for any longer amount of time than it takes them to pee, the coats go on. I have several, and the best are the ones that are easy to get on them, because believe me, they see me coming with the coats and you would think by their reaction that I am going to chop their legs off. They HATE putting their coats on.
As for bigger dogs, I have a friend out near Pittsburgh who has many mastiffs...she does rescue, but has over 10 that live with her permanently. And she puts coats on them. I guess it mostly depends on the breed, the dog's health and weight too...but it can be preference as well, or just wanting a dog who stays outside for a long time to be comfy.
Here is one of my dogs, Oobi, modeling one of the best coats out there...called a "Fido Fleece" They cover everything, entire belly too, have a little turtleneck and go half way down the front legs. A little of a pain to get on, they velcro up the back, but definitely the dog wearing it stays toasty warm!
