Ignorable assertions and interoperability

Hi

During the latest concall it was recommended to advise not to use ignorable assertions if the interoperability would be affected...I thought it was a strong statement at a time. 
The reason for that was that I was assuming at a time a WS-Policy level interoperability was referred to.
Most of the time it's obvious what interoperability the spec/primer texts refer to, but I feel it would be useful to revisit (in the primer and guidelines) all references to the 'interoperability' terms and qualify them as appropriate...

Cheers, Sergey Beryozkin