I don't see any advantage to a one piece over an extension. Having the extension offers the versatility of having a +2 up to a +5 or more magazine depending on what the user wants. With a fixed magtube you have what you get.
I own a couple extensions for my 870's. I've tried them and really just prefer to keep the standard mag, 4+1 works for me and makes for a quicker pointing lighter gun. I suppose if I ever really felt the need I could quickly set up 2 of my hunting guns with shorter barrels and 7+1 mag capacity. In addition to the longer field barrels I also have a 21" turkey barrel and a 20" rifle sighted barrel.