I blanket Tanner because when he starts shivering weight starts dropping off and it is HARD to put back on him. Of course he is a geriatric TB who never spent a winter on pasture before. I think hay and his winter coat should take care of Milo, but if he is shivering and his coat isn't up to par, maybe a rainsheet or turnout with some fill will work. Personally I think 30 isn't *that* cold but if he's wet and the temp drops it could be a problem.
I strongly disagree that a shivering horse isn't in danger, though. You have to consider other factors such as breed, health status, age, etc. A young fat quarter horse who has spent his life on pasture will probably do fine. An underweight arabian or TB who is older or doesn't have the best health should get a blanket for extra warmth.