The use cases of why to store the mappings in Wikidata itself (rather than are mentioned in our issue here:
Where Dan summarized and updated this in January basedon Lydia's feedback.  And its also much easier to express and maintain mappings in Wikidata :)

So that is 2 votes so far to use the Statement "exact match" P2888, and not link the concepts between the two using the Identifier construct.

I await a few more thoughts and reasons why NOT to use an Identifier?  I am looking at the nice hierarchy here on talk for Identifier and my personal thoughts are that Type and Property can be looked at in 2 ways in Wikidata, as an Identifier and as a Concept, so.... ?