My lurker's interpretation is that there are two model of emacs usage at odds here:
1) emacs is my environement (My emacs controls the horizontal and the vertical. OS themes be damned! Just stay out of its way.)
2) emacs as a good citizen in a OS themed setting
The latter is likely the expectation of (most?) modern newbies. They use many windowed applications that display text and perform simple manipulations using a foreground color, background color and selection highlight color specified by an OS theme. They may well have exploited their ability to adjust the OS theme. But being newbies they have yet to embark on customizing emacs' colors. Hence they expect fontlocked text to cohabit comfortably with their OS theme's background and selection colors.
/john