a heck of a lot of fly rugs on the market these days seem very gappy, and so flies can easily get under the rug and annoy the hell out of the horse. i've read a few horror stories on here about such situations. also, i'd be concerned about overheating as i've a very dark horse. as with all things to do with horses, you do what's best for the individual. my horse doesn't wear a fly rug (but does have a mask). my youngster wears a fly rug no probs, but he's much lighter in colour so doesn't overheat.