The dagger knife could certainly deflate a dinghy. With the advent of hard seat packs there was no need, and indeed you could not, deflate a dinghy pack. Well, that is what they said.
The banana, OTOH, was safer in the hands of children and could be used to cut parachute rigging lines if you got snarled up.
Theory.
Practise.
The knife was very good at opening tin cans and tinnies and could still be used to deflate the soft dinghy packs as carried in the Shack etc. If used to cut rigging lines the first thing you had to do was cut the knife lanyard as well.