You bring up a very valid point, I think. While my article directly addresses accents that practically no American has any trouble understanding (Don Champion is articulate and well spoken), I will never forget my freshman calculus class.

The teaching assistant who lectured most of the time was an immigrant with an accent so strong I could not understand him a good deal of the time.

Even after an entire semester getting used to his speaking, and making sincere efforts to learn how to understand him, I still had a lot of problems.