The flag just makes media-libs/libmp4v2 install, but as amarok uses media-libs/xine-lib, playing aac streams is not possible, unless flag is also set for xine-lib.
*** Bug 154114 has been marked as a duplicate of this bug. ***
*** Bug 154117 has been marked as a duplicate of this bug. ***
You misunderstood the meaning of aac useflag on amarok: it allows to read metadata from AAC/M4A files, has nothing to do with playing them.
From /usr/portage/profile/use.desc: aac - Enables support for MPEG-4 AAC Audio. It depends on how you understand to `support' a codec in an audio player, but I get your point.