@Harold This is a generalization, but what I find is people who live in places with many accents are more likely to be patient and listen more attentively to a foreign accent. My statistics professor had a string Indian accent, but I had no trouble understanding him. A lot of people in my class complained about his accent, and said they could not always understand him. They usually were from small towns, and rarely exposed to anyone who was dissimilar to them. Someone living in the American deep south, in a small town, with a strong southern drawl, who speaks very slowly, might have trouble with an English accent, they have trouble with some American accents. And, vice a versa for that matter. My husband had to print up a list of what not to say for his staff who answers the 800 line, because people calling in from other parts of the country would not necessarily understand what they are saying.
I do agree with you, everyone should make an effort. I also agree generally Scottish is more difficult.