Well, in southern California I would guess that it is usually fairly warm, then you have a few really cold days. On those days, I would blanket ANY horse in ANY condition because if you let the horse grow in a think winter coat, they will be miserable when it warms back up again.
That is the way it is in Florida. Those of you in really cold areas can go without blanketing because the temperature doesn't change so rapidly. If we in the south let our horses grow winter coats, they are miserable for many days! The thick coats do not leave as easily as they come! Today it is about 55 & sunny, but just 5 days ago the high was 27 degrees. A horse cannot adapt that quickly to the changes. It could be 65 in two days, then the next week, 18 degrees! So that is why the horses in the south get a lot of use out of their blankets...the "natural" way is not always the best way...
My horses wear something nearly every day, year round. Most of the year it is a fly sheet to protect them from the sun. Many people just change to turning the horses out at night & keeping them in during the day, but then the barn can get hot so you need to run fans all the time. By February it will be in the 60's most days, but we can still have a hard freeze in April.
The weather we have here in Calgary is very extreme, far more extreme than what you're describing and our horses adjust just fine. We can have sunny, warm t-shirt weather with the sunshine and go into a sever winter storm, where it's -20 in a matter of a couple hours. It's a pretty standard set of weather conditions we get here. How do you think horses have managed to survive the last few thousand years?