I'm inclined to agree that it's use would be uncommon, I've not encountered it in 30 odd years of reading papers and talking math (or rather I don't recall noticing it's use).

What I must point out is you make a weak case based on myopia- for example; in 15 years of working with tens of thousands of hours of high resolution radiometrics I've never once met and talked to a dendrochronologist who's looked at the radiometric signature of individual bands of Japanese pine trees.

It's my understanding that such beasts do exist.

Another math major here (in my final year). My professors routinely use square brackets after the second or third level of bracket nesting.

I think it must depend on the professor/university.

It's common in engineering. I like it because it makes matching parenthesis much easier.