They are a little more expensive depending on what pieces you are using. We had lots of problems with the threaded fittings leaking on our water truck, and it's usually not easy to get 3" stuff torn down to fix the leak whereas with these there is rarely a leaking problem and if there is, take it apart, put in a new seal and you're done.

Also, if you're like most guys, about every two or three years you want to change the truck around anyhow and with threaded fittings they are about impossible to get sealed back up then.