I believe this is very important to consider when thinking about how we wish to display our content. In this passage Klienman discusses how our fonts and typefaces are similar to the clothes we wish to wear and I believe that this is true.
This reminds me of something I've learned/overheard many times from my wife: don't use Comic Sans. Anytime someone uses Comic Sans, the author will lose credibility. Unless writing an actual comic book, the font is useless. In my humble opinion.
I believe that when working in the professional world, especially when that work uses design in anyway, the technical writer needs to understand how the content they are creating looks. If that content looks unprofessional, that content will lose credibility. So by having a firm grasp on fonts and typefaces we are working towards implementing better design, more effective content and a brighter future.