Blanketing does not take anything away as far as thickness or thinness of winter hair is concerned. Blanketing DOES "de-fluff" a winter coat so that's why it's important to, when blanketing, replace hair pile with fill - ie blanketing appropriately for the weather (no lightweight blankets in freezing weather, etc).
The hair coat of a blanketed horse in winter will look sleeker than that of an unblanketed horse but that's just because the sleeker horse is not "fluffing" to make him/herself warmer. Put a blanketed horse out without a blanket for an hour and he/she will very quickly fluff up and look like his/her unblanketed herdmates, coat thickness-wise.
2. I, in the past, blanketed my mare extensively. Last winter I even clipped her - therefore, she has a large selection of blankets that she used regularly! Haha
This winter, she's been recuperating from an injury and I've been more financially capable of handling feeding her as much hay as she needs to stay warm, so I've been letting her go "naked" a lot of the winter.
She does get blanketed when it rains or is excessively windy. She's 28 and has some medical problems that are exacerbated by becoming chilled, therefore (since she's more likely to stand out n the wind and rain than she is to head to the shed for cover) in cases where she's in danger of becoming chilled (rain/wind), I don't mess around.
I do believe that it's "better" for most young horses with healthy immune systems, plenty of hay, and a run-in shed capable of containing all the horses in the pasture, to go blanket-less during the winter. But I think as long as blankets are thoughtfully thought out (blanketing appropriately for the temperature, checking to make the horse doesn't become too hot, checing to make sure they aren't too cold, etc) horses can do well with them, no matter the circumstances.