I do carry and use the 1.4oz Zpacks pack cover as a first line of water defense. It also actually saves weight in rainy conditions because a wet pack weighs many ounces more than a dry pack. The pack cover, being DCF (cuben) does not absorb water.
An inside pack liner does nothing to keep the actual pack dry.
BTW, the zpacks pack cover DOES include the drainage grommet OEM.