I travelled with Siebrand the other day and I learned that in order to provide plural support at translatewiki.net, he uses the information in the CLDR to know what languages need plural support and in what way.
The amazing thing was that for some languages the plural support in MediaWiki is different from the one indicated by the standard. There are also a number of languages where the CLDR did not have information about their plural support.
It is vital that the CLDR and MediaWiki agree on how to provide plural support for languages. The CLDR is the standard and should be complete and correct because it exists for any application.