It gives added feeling of security. Mine has been in two different trucks, both with shells with no windows. Currently it's mounted on a cargo glide so the entire vault comes out. I did this to have dead space under the vault so I can use the tailgate to work on guns, yet still open the drawers.
We've stayed in hotels with the guns locked up. I feel comfortable in knowing people can't see what is in the bed, if the vault was visible, I wouldn't feel comfortable.
Cons, they take up a lot of space, however I can still sleep in the bed if needed.
Tully