But I would be utterly fascinated with a native Python control for InDesign. Some of Python's constructions seem very alike ID's Object/Collection constructions, and it has a powerful set of collection operators.
The client is interested in a specific library, not in the language itself. I think that the available libraries are the major advantage over at least ExtendScript - for example CPAN has 27982 modules, and I didn't bother to check github.
Of course npm right now claims 175143 packages for node.js, but CEP is not available for CS5/CS6. Besides even though I really like node.js I doubt that it has anything like AFDKO (Adobe Font Development Kit), and that's not the library in question.
Speed could become another advantage, but it is too early for any claims.
Hmm, Python? Sounds doable, but that would be a different weekend. Probably I'd first revive my old Java binding.
I'd second Jongware's idea about Python. The possibilities that that would open are amazing. Powerful string formatting and manipulations, socket server/client etc that can actually be used, multithreading!!