One of the observations over time wrt library standards -- I am thinking mainly of protocols -- has been that we have overdesigned, ending up with solutions that are complex enough to cater for every last case, but are not simple enough to be widely adopted. For this reason, they have remained within a library niche, and, in some cases, have not been widely adopted even within that niche.
A version of this discussion emerged recenly in relation to Z39.50 and the related, simpler, SRU/W approaches which reimplement Z39.50 semantics in a web services idiom.
The NISO Metasearch Intiative has been working with the SRU/W developers and has produced a proposal for the NISO Metasearch XML Gateway. As I understand it, the aim here is to further simplify the interaction between a metasearch application and a data provider by removing the need for query grammar interoperability while retaining the ability to return structured data which facilitates manipulation and representation. In this way, the cost of implementation is lowered at the expense of interoperability across multiple data providers: each data provider expects a query in its own query grammar.
Currenlty, the Metasearch XML Gateway defines three levels. Levels 1 and 2 are each a non-conformant subset of the NISO SRW/U standard. Level 3 adds capacities which make it minimally conformat to SRU.