My thoughts on blankets:
They're one of those downhill slide things. You get one blanket, start blanketing your horse, your horse gets used to it and becomes a big baby about not being blanketed, blanket gets ripped, you end buying another one, then you decide that maybe you should get one of each weight so the next thing you know-you have 3 blankets and one ripped one, then one of the new ones gets ripped, etc etc etc.
It won't necessarily hurt your mare if you're blanketing correcting depending on the weather. Look up "blanketing chart" using Google for more info on that. If you aren't blanketing correctly, aka you're doing something dumb like using a light weight blanket in 20 degree weather, or something similar, you can certainly harm your horse and cause her to become colder than she would have otherwise.
Some people believe that blanketing will help stop the winter coat from growing in. Personally, I completely do not believe that. It certainly helps the coat lay down flat so it looks like there's less hair, but if you leave the horse unblanketed on a kinda cold day after blanketing him/her for quite a while, your horse will suddenly have a normal (for him or her) winter coat, once the hair fluffs up. My mare has a winter coat similar to that of a yak and her hair still gets to be 5 inches long, and thick, all over her body, even though she's blanketed 24/7 during the winter.
A blanket will not cause your horse to loose her winter coat either. Only being put under lights and having her body "tricked" into thinking it's spring, or having it be spring, will do that.
My criteria for blanketing is:
Age of the horse (older, 20+year old, horses, yes. Young healthy horses, no.)
Condition of the horse (skinny, any age horse=blanket, fat younger than 20 horse= no blanket)
Availability of shelter (no shelter at all=blanket, shelter+young healthy horse=no)
Personally, if I were you, I'd save my money and and see how she does this winter without a blanket. If you find that she seems uncomfortable, then start considering a blanket appropriate for your climate. I bet though that she'll be fine. :)
Also, she is very very cute. :)
Good luck! I totally get the "but this blanket is so cute!!" thing.
Fabio - 13 year old Arabian/Lipizzan gelding
Hazel - 14 year old Angora goat
Atticus - 4 year old LaMancha/Alpine cross goat
Rest peacefully, Lacey.