In other words, a much more important factor would be whether the person in question has a neutral accent or not.
I can better understand a Korean or Japanese person speaking English as a second language than I can understand a Scottish person speaking English as their native language.
The Scottish person will speak at a much faster rate and their accent will muddle their words for my ears. I have NO idea why you are so upset because some people claim to have a difficult time understanding other native-English speakers. Of course it has to do with ignorance... I've not had a chance to live in Scotland. That doesn't make me a worthless English teacher.
Dear God! Can one not have a simple debate on here without somebody invariably coming on and accusing them of being 'upset' because they're offering a different viewpoint. In terms of me personally, I have a very neutral accent and never had any issues about my accent in any country that I've ever lived or visited.
Anyways, back to the issue at hand and your example of the Scots. Again that often depends on the individual themselves and the region of Scotland they hail from. (You also have to factor in who the actual listener is and what part of the world they hail from). I know people from Scotland who speak quite clearly and are very easy to understand. Not all Scots are as you describe and again there's always variations within countries. Listen to the tennis player Andy Murray in the video below. Would you also claim not to be able to understand him?!
http://www.youtube.com/watch?v=MiHokF5dqGsFinally, I never once disputed the fact that some dialects are more easy to understand than others (dependent again on the who the actual listener is) and the only thing I was pointing out was the ridiculousness of some native speakers claiming a '
total inability to comprehend the accent of a particular country based on their experience of talking to a few native speakers from said same country'. Nothing more, nothing less!