the only reason I can see for a horse to get skin problems etc with a rug is cause its dirty on the inside and so are they on they outside??
My horses each have a sheet underneath and their rug/s on over top, simply because its amazingly easy to keep a sheet clean and they don't get irritations.
My horses get their rugs off daily, unless its cold as a frogs tit, or pouring with rain, they get let out of them for a bursh and roll so they don't get itchy
R.I.P ~ Bubbles - 25yo tb mare - 13.04.2011 ~ 8:30am ~ passed away naturally and peacefully in my arms