Hydration pockets are good for day hikers because they hold your Camelback or platypus hydration bladder in place. If you’re a day hiker, you don’t have a lot of gear in the pack to keep your bladder from shifting around. However if you’re doing a serious hike and you are carrying a decent amount of gear, it is near impossible to get a full water bladder back into the hydration pocket when you stop to filter and resupply your water: you basically need to unpack your bag and repack it from scratch. It’s not worth it and serious ultralight packs don’t have a hydration pocket for this reason.
My serious pack is a Six Moons Starlight (25 oz) and it doesn’t have an hydration pocket. When I pack it, I always place a full 3L platypus in the upper right side of the main compartment to counterbalance my tent or hammock and shelter accessories which all go in the left outer pocket. When I stop to filter water and refill the bladder, it is easy to to put a full bladder back into the pack and off I go. No need to repack.