I agree with the author that turning syntax off is like reading a book. When I read in English, I don't want to be bombarded with color or style changes that aren't completely necessary. I appreciate their function as an aid to recognizing patterns, but I've read enough so that I can glance at the entire page and recognize all the patterns I want in a few seconds. Now, when it comes to reading books in a foreign language (e.g. japanese or mandarin) it can be a tremendous aid while you're learning because it helps you make sense of where the breaks are in something that might otherwise take a considerable amount of time to break down bit by bit. As soon as you start getting used to it the patterns become obvious on their own, and the visual aids just become a layer of noise.

My main exception is comments, or footnotes on the side. At a quick glance it's very helpful to skim through a file and see what's documented or, if it's a text/config where each section break is.

Edit: after skimming the other comments here it seems that the preference for highlighting is more to do with error checking than anything (pairing enclosures, etc.). If that's the main concern then why not zap the rest and use syntax the same way you'd use spell? I don't use highlighting for much but I might just drop that into my vimrc tomorrow to play around with it. I would rather see glaring red blocks where there are mistakes than a minor variation in my digital bowl of lucky charms.