We have seven thoroughbreds and all of them live out 24/7 365 days a year. They range from a 6 month old filly to an 18 year old mare. All of them do just fine. However, in the winter and summer they tend to need more food because in the winter they burn of huge amounts of calories staying warm and in the summer they burn off even more than that sweating/keeping cool. They all seem to enjoy it though since they can graze at whim and run around if they feel like it. I find that it helps keep ulcers away and keep them healthier over all and wouldnt change it unless I absolutely had to.
We also don't blanket ours (except for the 18 year old) in the winter, but we live in Texas where it rarely gets below 20 degrees or so even at night.