I don't like the internal water bladder pockets on rucksacks, a, they take up space that could be better used for something more useful. b, Platypus's do leak from time to tome, so you'd end up with water inside your pack. c, They're a pain to get your platypus out of to refill.
I have mesh pockets on my pack so I just stuff mine in there and thread the tube up over my shoulder.