You would just scroll left or right so that the effect you want is highlighted within the 4 slot "window". The "window" isn't locked in a static state. Granted, the example you cite REQUIRES scrolling, but that is not to say you have no reasonable access to the front end comp. You just may not also be able to access the other FX readily, and scrolling is required.
You're quite right, i could scroll everytime I wanted to switch a particular effect in and about, but you have to admit, bending over every few songs to do so isn't exactly convenient.
To be, the idea of a foot controlled pedal/unit, is just that. Foot controlled. If well designed and setup correctly, you should be able to play a whole set without bending over to interact with it.
I say this, because in my regular effect chain, I have a couple of always on effects, plus 3 others that I regularly switch about in and out, and another 1 or 2 for specific songs. Yes, I would set up specific patches for those specific songs, not a problem, makes perfect sense. But for my core sound, I want one patch, with several effects in a particular order and stomp box control over 3 of them. I do not want to make duplicate patches, as if I need to tweak something, I need to do it for each patch (I have to do this with my MS60B and it's a pain).
Surely it not beyond the realms of possibility to let the user choose which effects are 'live' and which are hidden, rather that being forced to order them in a certain way so they show up together?