Altough browsers currently zoom better whether you use px or em, not all visitors know how to zoom or are used to other tools. Furthermore, what if you want to zoom the text but not the entire site? For instance, there are some sites which have buttons to enlarge all the text. In those cases by using em, you only have to enlarge the base font-size, and all the text will scale relative to that (but without scaling the entire site).
And you can argue if it’s a waste of time for such small a group. Yet, how much extra time does it take? If you know your base font-size, how much work is it to calculate it in em? After you have done this a few times, it will just get easier. I am used to it right now and using em takes just as much time as using px.