More and more fonts are available with more than the 4 legacy regular/italic/bold/bold italic styles (dejavu is such an example)
If you make such a font the default "Sans" "Serif" or "Monospace", however, only the 4 legacy styles will be exposed via the "Sans" "Serif" or "Monospace" aliases (tested in GNOME's font selector)
Fontconfig should not filter styles in such a way.
This is not a fontconfig bug; the gnome font selector would have to convert the alias into a family name and then list that family to detect which styles were available.
Moved to http://bugzilla.gnome.org/show_bug.cgi?id=490609