Having the ability to access teammates backpacks would be great. Benefits would be that...
Item transfers would be easier instead of just throwing stuff on the ground and having someone else try to find it.
Grabbing ammo/mags/meds while the other provides cover fire.
Particularly when a teammate is down, they can store an "IFAK" of meds for a teammate to pull meds from to heal the downed individual. This would add to the realism since soldiers generally carry a personal IFAK that is used specifically for them and no-one else.
To help mitigate trolls, a "Backpack Access" setting could added with options "Closed", "Squad Only", "Faction Only", "Open".