I had a pop-n-lock, but I also got a shell with a lock tied into truck's keyless entry. After about a year I realized I hadn't been locking the tailgate. With the Leer's I can't get the tailgate open, but with the tailgate unlocked it moves a bit when you try. I suppose someone might conclude that if the tailgate moves a little, they can force it open.
However, on the Leer XQ (previous truck) or XL (this truck) there is nothing to grab on the shell - just a little flat piece of plastic that twists. No handles.
So, I suppose someone COULD force the tailgate open. But they could use a crowbar on the shell, too. The shell with a glass door is still limited security.
But I feel the same way about convertibles. Then again, my friends who have had convertibles broken into didn't have someone take a knife to the soft top. They just broke the passenger window.
A Tonneau cover that locks is going to be more secure than any shell with windows I'm afraid.