Ahh! Gotcha! the thing is, the raincovers are meant to be used when the bag is carried, so one side (back of rucksack/back of pouch/back of shoulderbag) is not covered by the raincover, so you would have to face it into the wind, so to speak. When out carping with my Son, I always take a large, sealable plastic bag with me for the most terrential rain, but you need to take care that you don't get moisture trapped when you seal it.
Must say though that I very seldom have to use the plastic bag, for most situations the LowePro cover is fine!