What follows is not a rule; it only reflects my own experience, and is not intended to prove anything or “solve” the discussion.

In my own 35 years of experience in the woodlands of the eastern USA, I’ve never relied on anything beyond the pack’s own integrity and a pack cover for waterproofing. I’ve used cheap packs (Texsport, an 80s version of whatever WalMart sells these days), Dana, Gregory, Lowe, Osprey, Granite Gear, and Deuter, with a variety of roll-tops and drawstring tops (with and without lids.) I’ve hiked in some day-long, boot-squelching, no-raingear-works hard rains, and I’ve never once had any problem whatsoever with anything in my pack getting wet. Not even a little. The pack itself got wet, sure - on the shoulder straps, hipbelt, and other areas not covered by the pack cover. But inside the pack, never a problem.