Since secondary types (Compilation, Live ...) were introduced for release groups, it's been possible to create a new release group that has no primary type, just a secondary one. That is, instead of "Album + Compilation" or "EP + Live", the group might be just "Compilation" or "Live". While there might be some small utility in this when it's not clear what the primary type is, more often it just fragments the display of albums on the artist's page with a few release groups under "Compilation" or "Live" but most under "Album + Compilation" or "Album + Live". This seems to arise mostly through inexperience, I think, as I've usually come across it in situations that are clearly Album releases.

Musicbrainz should not permit new release groups to be created without assigning a primary type. If that seems too severe (and we want release groups where the editor can't identify the appropriate type or feels none of the types apply), then at least warn the editor that a type should be assigned, to avoid inadvertent creation of groups without one.

Example http://musicbrainz.org/release-group/0bbb771e-f0b7-4e5c-ab2f-3c5cbddf4be4 is tagged 'Live' (not 'Album + Live') yet is a 2 CD official release. (Neil Young's 'Live' albums are bootlegs, apparently, perhaps one situation where it might make sense to drop 'Album').

Another example: http://musicbrainz.org/release-group/34b2dc86-dac3-37ba-9702-f6977545c262 is one of three Beach Boys compilation albums, which seem to be official releases in this case but are not to be found under 'Album + Compilation' because the primary type was not specified. This is a good illustration of why my improvement would be valuable; it's easy to conclude that an album isn't already entered because it isn't under the expected type+subtype heading.