I prefer to number only the important equations that the reader really needs to know and focus on, and remember. Thus appearance of the equation number signals to the reader "this is important." If I were to number every equation, I'd be losing this communication signal.
In fact I find mainstream textbooks (for undergraduates) to be really overwhelming with the visual elements (colors, boxes, extra emphasis). If you flip through a physics or a calculus textbook you'll see all kinds of boxes that call for your attention: THIS IS IMPORTANT, THAT IS IMPORTANT TOO, etc. one on every page, but in the end when everything is important, nothing is.
In fact every chapter has one or two main equations that you really need to know. Put those in a box, and number them if you want, but leave all the other "anecdotes," derivations, and special cases unaccented please.