think some people are underestimating the gear you need for trad multipitch climbs
1. single rope … ~10 lbs … possible tag line or second rope carried by partner another 5-10 lbs
2. cams from 0.3 to #3 camalot, 10+ slings with 2 biners each, 1 full set of nuts, belay device, harness, rock shoes, helmet, cordelette and 3+ biners for anchor material, nut tool … 10 lbs + … possible doubles depending on the route and even triple in some cases … so very easily 15-20 lbs_ …
even split with yr partner that and the ropes is easily 15-20 lbs each just for climbing equiment
3. water … 2-3 liters there isnt much refilling in the middle of a full day climb … 4-8 lbs … food … 2-5 lbs … jacket .. 1-2 lb … essentials 1-2 lbs
you have 3 choices on a climb with yr pack
1. leave it at the bottom … bears call this a pinata … pray you dont get benighted
2. climb with it on yr back … better be durable if you need to chimney
3. haul it …. better be REALLY durable
NO MESH pockets … if the rock doesn't shred them, the gear will, climbing packs get trashed .. i consider 200D+ dyneema gridstop to be the minimum … many cragging packs are made of 400D to 1000D+
and this is for a SINGLE DAY trad climb with a LIGHT alpine rack
it easily gives 25- 30 lbs person … all that weight needs to be carried up a possible 2-3 hours bushwack approach … and then for 10-20 pitches and then down the rappel or descent
the cams will be digging in yr back if you dont have a good pad, frame … there isnt any easy way to pack all that metal gear without it collapsing yr pack if you dont have a frame/pad which is why a platy and holder comes in handy
make that a multi day alpine rock climb and yr looking at 50+ lbs starting easily
and yes alot of climbers do lighten up the other stuff as much as reasonable … there are some things you cant just get rid of without taking a lot more risks in terms of safety … especially beginner/intermediate climbers
now granted you can carry alot of gear out of the pack if needed … but yr more likely to lose gear, trip up on yr own gear (bad), damage gear, and its a b-tch on long approaches
back to the OP's question … a 35-50 L climbing pack in the 2.5-3.5 lb range would work well for most other activities as well … most good climbing packs can be stripped down to a minimal weight by design … so IMO climbing packs can be used for anything
the question is how much serious climbing will he be doing?