That's the most non-sensical thing I've ever heard. So it must be true. xD
My youngest brother's canteen doesn't have a cover and that thing warms up quick. Drenching the cover is a necessity to keep it cool. Also, no burning yourself, there's less noise, and no blinding friend and foe alike... so many pros to having a cover.
By the way, sweet canteen there, Dordak. I'll post my ACW, WWI, and WWII canteens soon enough.