I think this one should be gzipped.  The spec states that the identity encoding is always acceptable, but doesn't state that unless otherwise specified, it should have the highest priority (ie q value 1).  I think anything defined here that doesn't have a q value of 0 should take precedence over the identity, unless it's explicitly stated.

So, in practical terms, the default q value for identity should be 0.001 (that being the smallest permissible non 0 q value), which means acceptable, but lowest priority.