As I discussed in a comment on http://codereview.musicbrainz.org/r/1875/, it shouldn't be – or, alternatively, the decision to set this primary needs to be documented.
In fact, it looks like Audiobook is in both lists.
Can we have some examples
examples can be found in the release editor of any release group: Audiobook appears in both primary and secondary types.
IMO it should be only primary, unless someone can think of why there would be an "audiobook EP" or "audiobook single" (or even "audiobook album")
audiobook single: http://www.amazon.com/Go-the-F-k-to-Sleep/dp/B0055QAEVE/ref=tmm_aud_title_0 / http://www.amazon.com/Go-F-Sleep-Adam-Mansbach/dp/145584165X/ref=tmm_abk_title_0
It's 6 minutes long.
So which is it? Until this can be decided, I'm not working on this. "IMO" is not a decision.
Make it secondary. That's what the original "split the types" ticket said, and a change should be an RFC.
Yes, it should be secondary, since that's what the original ticket said. Further changes can go through style; this bug is strictly about a mistake in implementing the original ticket as written.
In review at http://codereview.musicbrainz.org/r/1899/