If they learned the mechanics of writing.
I thoroughly enjoyed this brief glimpse at your writing skill. I hope you find other reasons to post messages.
I do believe your students are fortunate to be taught by someone with your obvious skill and experience. I'm also fairly certain, you've come across at least a few bright people who were in desperate need of your help. You may even have stumbled upon books on library shelves written by brilliant people in desperate need of a copy editor.
Having worked with a few bright people who lacked basic writing skills, I know the evidence you ask to see is easily found. The scary thing is some of those people hold advanced degrees in their fields. That should tell you what most interested the universities which granted them the degrees.
Thank You! This is a good starting point.
http://reading.indiana.edu/