"May", "Might", and Other Difficult Examples

In his essay "Illiteracy and Other Ills", Jim McCue argues that an increasing part of Britain's and America's population is unable to write as well as talk coherently and grammatically correctly. In general, I found the text quite interesting, as I myself am rather critical when it comes to incorrect utterances that I hear or read—I find it hard to not comment on other peoples' mistakes and always try to write accurately, even in an informal context.
Particularly interesting to me appeared the author's argument that "neglecting grammar is also neglecting constitutional structures" (p. 14). Although it first seems to be a little far-fetched, I think he has a point claiming that especially in politics, wrong grammar is the key to confusion and inappropriate assumptions. After all, how can you precisely express your position if you do not have a competent knowledge of all of your language's possibilities and rules?
Generally, I found it hard to comprehend McCue's numerous examples though. As a non-native speaker of English, I could not spot the mistake in sentences like "he didn't so much rise to power but leap to it" (p. 11) and do not know the exact difference between "may" and "might" myself. Which leads me to the question if the allegedly illiterate population of English-speaking countries really is so much of a problem. Maybe the overwhelming majority of non-native speakers in the world presents the real danger to the English language. Maybe they are the ones causing proper grammar to fade away once and for all—who knows?