The point of the compiled in system font is to have a fallback in case of error - to get error messages shown even if e.g. font loading already fails. You should actually never see it.
A font that is compiled in makes the whole firmware file bigger, that's why a small font is chosen and also with a smaller character set, just enough to display error messages.
The fact that you can use the system font in the WPS is just an offer for at least one other font because it was easy to do, more or less a byproduct.
The correct (and wanted) way would be to have 2 or more user defineable fonts loaded from disk , known as "multi-font" - which of course is more complicated than one loaded font + the compiled in. Unfortunately no-one did the hard work of implementing this in a non-hackish way, there is just talk about how nice it would be...