I definitely understand the security issues, though that isn't a flaw in the product per se. The dust is a bit of a concern, however, since I live in UT and most of my off-roading is done on dusty ass roads.
My reason for wanting one, is I don't want a cap/topper 90% of the time because I hate the way they look. I also want to be able to use my truck bed to do truck things. However, when I go duck hunting, I don't want my dog & kennel out in the open on the highway. Or, for camping trips and ****, it's a lot easier to carry all my gear without having to worry about tying it all down. Plus, it'll be nice to pop it up when I drive to hockey games, or whatever, and get caught in some kind of inclement weather.
I do think if you're using the bed as storage all the time, it's probably a poor choice; especially if you live in an area with a higher property crime rate. But I also can't think of any other option, when one really only needs a cap occasionally.