Question to Bellows Users

Is there any advantage in having a macro Bellows set up mounted on a separate rail so you can move the whole lot back and forth for focus, or you better off with just the bellows rail?

There seems to be both with and without configuration for sale, so wondered if you could point a beginner in the right direction.

The basic bellows that simply extends the distance between lens and camera is a magnification device. You can tweak the focus with it but you will change the size of the object too. Now put camera, lens and bellows onto a separate focus rail. You choose the magnification with the top rail/bellows extension; then you move the entire sheebang back and forth, using the bottom rail, to focus. Say you were doing a Coker style 30 shot series for a focus stacking job. If you relied on the single bellows rail to move the focus along the subject it would get bigger and bigger and bigger (or smaller, depending which way you go). You need them all the same for this caper so the two rail system is better as you only shift the focal plane, rather than change the object size.
The Pentax ones are very nice but the traditional BPM products are just as good, so there's a lot of choice out there.