pet peeve and its reflection

I was about to bitch about how students seem to be entirely clueless about spacing when it comes to hyphens, dashes, equal signs, etc. Then I found myself on a usage page which -- and here I illustrate -- tells me that my own usage of em-dashes is wrong; apparently you're not supposed to put spaces around an em-dash. Huh. Well, I still think my students' mistakes (poorly- spaced like this) have a worse effect on legibility than mine do, and in fact think the usage rule I'm violating is dumb. So there, usage guys!