That is why most magazines and scientific papers use columns! Otherwise, why would they do this instead of just using a 9pt font with a line going from one paper edge to the other keeping the margins below 1cm? Why would newspaper even use more than two columns?
Usually, I keep KOMA script margins recalculated with DIV=calc using the current font (bookman for example is wider than times: they need different margins).
If the document is to be read mainly on screen, I stick with the default one column format. Columns force people to scroll back and forth on each page which is annoying.
If the document is to be printed several times, I set up my document as two-column. It is not annoying/tiring for the eyes, you increase the ink density on the page while keeping the line at reasonable lengths.
If the document will be printed once, and read mainly as a PDF (such as a PhD thesis), I would go back to the one column format.
I would like to lay the emphasis on one fact:
I calculated the average number of letters in my own PhD with the one of another student. I was using LateX with KOMA script. The other was using MS Word (narrow margins, double spacing). Believe me or not : the average number of letters was similar! One a random page with text only, I could fit more letters with KOMA-script that he would do with Word.
Now the question:
If you want to tell the people that LateX follows the best practice in typography, have them read the manuals and read this study for instance. Make them opening a book/manual and count the number of character per line.
But actually, as said before, in most cases, you are screwed: these people are not ACTUALLY interested in what you could say. They are used to a certain layout (dating back to the glorious era of typewriters, when dinosaurs were still setting foot on earth) and that's it! At that time (typewriters), you were working on a page to page basis. If you had to change something in your thesis (after the presentation), you had to do it with a pencil, then unfold the thesis, then type the corrections where you can (even if it means adding a page) and fold back the thing. Double spacing was needed for the jury to correct the thesis BUT also for YOU to type these corrections without having to retype everything! At that time, you also wanted to fit as many character as you could in one page... Yes, this ugly layout was used because you technically HAD to. Now, it is difficult to convince people that what you are doing is good practice (I had even to convince my supervisors that LateX was not "just for fun" because they didn't know it!).
Well, if you want to keep or share your document using LateX defaults instead of double spacing and narrow margins, it's just 2 comments away ;)
Then uncomment for these "people" you speak about.