I swore that the next time I bought a new truck, it would have a cap again. I always had caps for my trucks with the exception of a couple and having a cap was infinitely more useful than not having a cap.
When I was younger and more apt to move refrigerators into houses and things like that, sometimes it was a love/hate relationship with the cap, but these days anything too big to fit in the back of my truck with the cap on is being delivered!
Major bonus to the cap is that you can camp inside it if you really wanted/needed to, and you can put your muddy dogs back there without worrying about them getting hurt in an open bed. The other stuff has probably already been said - I like being able to leave all the shop tools in the back at the end of a workday and not have to truck them in and out of the house every evening, locked up and secure (I don't live in a dangerous area anyway, the road ends at my house), and I can go to lunch with a bunch of supplies in the truck.
If I need to move something that needs the cap out of the way I have two good options: first, I can go to my cap store and they'll take it off and store if for me for the day/week/month if necessary. Or I can rent a trailer for the day and just do it that way.
My general experience: anyone that's not a farmer that seriously uses their truck for work has a cap. Those who use their truck more for recreational purposes (carrying an ATV, motorcycle, etc) prefer having an open bed. I've owned trucks all my life and on balance caps are more useful than not having a cap, especially given the height of the bed on full-size trucks these days where putting big, heavy things that high up is hard on your body and more dangerous removing.
Open truck beds are really good if you're a farmer, because you need to move a lot of dirt/hay/manure and you can mound it, dump it with a front end loader, etc, but beyond that the lockable bed is way more useful. Yeah, the truck doesn't look as cool with a cap on it but I'm too old to worry about that ****
Side note: I see *a lot* more trucks with caps on them in the last 2 years than I saw in probably the previous 20. I remember a time where I was pretty much the only guy I knew with a truck that had a cap on it. Now I know a lot more people, and I see a lot more trucks tooling down the road with caps. They've come a long way since the one I had in the 1980s
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Leer color-matched cap with liner on my Maximum Steel Ram