These are the people, of course, who would take advantage of the free gate checking of bags but they've managed to pack something valuable into the bag and don't want to let it go baggage class.
I was with you up until this point. I'm one of those people who refuses to pay to check bags unless I absolutely have to. I'm also a stickler for the rules, if for no other reason that I don't want to give the airlines any reason whatsoever to make me pay for extra services I don't want. Because of that, my bag is completely within regulation size and weight limits, and I don't have a lot of sympathy for airlines who can't find a way to make my standards-compliant carryon luggage fit into the space it was designed for.
Furthermore, because it's now so expensive to check luggage, all of my daily necessities are in my carry-on. The fact is that sometimes the airlines lose luggage. Another fact is that I've never once lost my own luggage that I was carrying with me. Unless forced, I'd rather take responsibility for my own belongings so that I won't find myself going to a job interview with unbrushed teeth and wearing the same clothes I wore during travel.
There are plenty of selfish reasons to want to bring as much possible in a carryon. That doesn't automatically make those reasons bad or invalid.