Pocket knife popularity is a bit of a mystery to me as well, but in the US I figure they dominate because self-defense firearms are available (although the convenience of those seems dubious to me, when even a 5 shot aluminium J frame weights more than a 10 inch Rambo knife). Outside the US, in the rest of the Western World, crime is so low people carry nothing for defense, so pocket knives in Europe again win big (clip thumb opening knives are in the same category as pocket knives to me) because they are not carried for defense.
I have never found any use for pocket knives in my daily activities, and I cut stuff often... I use Exacto blades when I need to cut something, quite often just the blade without the handle... Most pocket knives have poor fine cutting ability compared to several Exacto-blades, especially when a fresh edge is needed after some previous work...
The rare times I ever needed a knife was in dire emergencies, usually related to a bike breakdown, where a pocket knife would have been of no use: I was glad I had a large fixed blade to leverage the chain back on, and once a Model 14 even gave me access to badly needed shelter...
I never got the usefulness of a pocket knife for daily activities, and I think Andy Rooney put it best when he did a 60 Minutes segment devoted entirely to pocket knives: "They seem like a better idea than they are."
Gaston