I have tried many different layouts (there were 6 revisions before that one).
A common use case for me is to have sources which have modulations (say from an envelope or LFO), mixed with sources which have no modulations.
With the Intellijel Quad, the loudness of the modulated sources has to be adjusted by their CV amount pot (the small one), and the loudness of the unmodulated sources has to be adjusted by their LEVEL pot (the large one).
It boils down to this: if you want to make an enveloped sound louder or softer, you won’t do this by offseting the envelope, but by increasing or decreasing the amount of this envelope.
This is why I normal the CV inputs with a constant voltage (instead of cascading the CV of the previous channel), and also why I use the larger control (here the slider) for the CV amount. With that layout, the level of a source, modulated or unmodulated, is always adjusted by the slider.
It’s a matter of affordance - the 4 sliders make the module obviously look like a mixer. It’d better behave as much as possible like one.