Boots rub my horse, yes all boots :P I'll use open fronts when jumping, but schooling I limit my boot use. They are great in warm-up arenas because they come off easier than polos.
I agree that polos keep the alignment of the soft tissues in the horse's leg, and although they don't offer much support or protection, they offer some. What are we doing to our horses that we need to bullet proof their lower legs??
I don't know how popular it is in your areas, but some places I've been people swear upon using Climatex under polos. Do you think this has any merit?