I don't think it is the fact that some of us can't manage without them, but there are some tasks that are much easier and safer to do using scissors instead of a knife.
Try to cut your fingernails with a knife...or those nasty hairs, residing in ones nose ;-)Seriously, better have it and dont need it, than need it and dont have it...
My teacher handed me a pair of scissors in kindergarten, haven't looked back since.
So why, if you have access to a knife, do you need scissors?
If in a public place using scissors where possible causes you less hassle than going all crocodile Dundee.