The description of "/mex:Metadata/mex:MetadataSection/@Identifier" contains the following:

"The interpretation of Identifier is Dialect-specific. While the Dialect attribute indicates the metadata format and version of the metadata in the Metadata Section, the Identifier attribute MAY be used to identify a Metadata Section or to just differentiate between Metadata Sections containing the same type of metadata. The value of the Identifier MAY be the same as the value of one of the attributes or elements of the metadata in the Metadata Section (if the metadata is included by value) or in the representation of a metadata resource (if the metadata is included by reference through Metadata Reference or Location). A metadata publisher MAY choose any value for the Identifier. The values of Identifier attributes in multiple Metadata Section elements in the same Metadata element MAY be the same. For well-known metadata formats, it is RECOMMENDED that the value of the Identifier comes from the metadata when that is possible, as the table below shows."

When you boil this all down it comes to "An @Identifier MAY be anything." In addition to this, for Dialect IRIs that WS-MEX defines, the RECOMMENDATION that the value of @Identifer come from the metadata is too weak.

General Proposaql: Rewrite the above paragraph to indicated that, if the Dialect IRI is one of those defined by WS-MEX, the @Identifier MUST match the value from the table. In other cases @Identifier is not constrained.