If you're still not sure whether or not you want to carry a rain cover for your backpack, the ultra-light and small packable Ultra-Sil Pack Cover offers a very good solution. It fits over most rucksack shapes and completely encloses the rucksack except for the back plate. So why not take a cover with you?
Is a very light waterproof nylon fabric. The outer shell is surrounded by a silicone layer; the inner layer is PU coated.