I wear shorts because it feels easier to run in them. Once I switch over to tights/sport pants, I feel it adds about 10 sec per mile of effort.
I usually wear shorts down to about 30 F. If it isn't windy, wearing them into the 20's is fine. If I feel up to a challenge, I'll take a shot at setting a personal low temp record, but I don't get the chance every year to go for it. Last winter I broke my old record by chance when it ended up much colder out than I expected at the end of a training run.
I only think about dressing for my own benefit. But I must admit that I kind of enjoy it when my Mother-in-Law freaks out because I might freeze my lungs when I wear shorts in cold weather. ;-)