Certainly with SXX, a dense negative should give good shadow detail, since the shadows will be well off the toe, and there's plenty of scale at the top of the curve to keep good highlight detail, as long as you're not trying to increase contrast through extended development, say, two zones beyond normal. Other films may just max out in the highlights before you get that far.

I wonder, though, if another factor may be at work having to do with Azo's reciprocity characteristics or something of that nature. A denser neg might just make it easier to get to Azo's ideal exposure time, whatever that may be.