Ctrl++ is only necessary because you've "thrown away the user's font size choice".
As a user, I have my font preferences set to what I want to read. If you maintain main body text size = 1em, then those preferences are respected. If you start arbitrarily specifying font size in px (whether CSS px or physical px), then you destroy that. This can still happen with 1em, too, by opting for a font whose em box is screwed up, as described in the article, but that's the _easiest_ thing to test for, because it will be apparent on all screens (including the designer's).
As a user, I have my font preferences set to what I want to read. If you maintain main body text size = 1em, then those preferences are respected. If you start arbitrarily specifying font size in px (whether CSS px or physical px), then you destroy that. This can still happen with 1em, too, by opting for a font whose em box is screwed up, as described in the article, but that's the _easiest_ thing to test for, because it will be apparent on all screens (including the designer's).