The search type selection area in the top right hand corner of the MusicBrainz page only shows "Artist".
Hiding the other search possibilities may have effects that the designers haven't fully considered.
Though maybe I'm mis-understanding the design brief.
EDIT (days later) : My fear that MusicBrainz UI designers had gone over to the dark-side of "contextually dependent", "mates club", "cliquey", "only for the cool kids" UI design has been laid to rest. This is not the case.
Rather there seems to be some compatability issues that result in the drop-down arrow not displaying on browsers such as Firefox.