Rucksacks are in a way the most troubling gear to choose, not because there are so few to choose from, unlike most other climbing gear, but because there are so many! Yet not a single one would be perfect. It is partly because the use varies so much, and different sort of activities demand different types of rucksacks, and partly because some demands are inherently contradictory, such as durability and (light-)weight.
After all a rucksack is just a bag on your back, and so you may argue you could manage whatever you have, as long as its volume is adequate for your use. It is true to some extent, but it is the same as claiming top climbers could climb an E1 with wellies. Yes, they could, but will they if they have a choice? No. Can they climb an E8 with wellies? No.
Rucksacks are arguably the second most important gear after boots or shoes in (alpine) climbing or mountaineering, as you use one all the time on your back often for hours continuously. That means the difference in rucksacks determines whether you can enjoy every moment or you suffer every second. Given you do climbing for fun ultimately (if type-II) and not for the sake of pain, to get a right sack is very important.
Also, with a wrong rucksack, your climbing ability and hence safety in mountains are marginalised, too, compared with otherwise.
Let’s get it right.