I've used a couple of packs with roll-down closures, though not the Pinnacle, and found them no worse or better than other, more conventional packs when it comes to waterproofing.
Packs tend to be made from waterproof fabrics, but because seams aren't taped, they will generally leak through the seams in heavy rain anyway. There are a few exceptions with welded and taped seams and Macpac's Aztec stuff has always been more water resistant than most, but if you want to keep stuff dry, my suggestion would be either to use a pack rain cover or a dry-bag style pack liner which - wait for it - will use a roll-over closure :-)
