For touring-focused riders, a backpack with no back protection is a perfectly valid choice and often preferred. The biggest advantage is weight savings—every ounce counts on long uphill climbs, and skipping the protector keeps your pack as light as possible.
That said, going without means you're relying solely on whatever you're wearing underneath for impact protection. If you're skiing technical terrain, sidecountry, or hitting the resort where falls are more likely, you might want to reconsider. Also, don't confuse a padded foam back panel with certified protection—foam is just for comfort and load support, not impact absorption.
If you already wear a separate back protector vest under your jacket, you can confidently skip it in your pack. Just be honest about the terrain you ride and whether saving weight is worth the trade-off for you.