Since the idea of UJS is loose coupling, the UJS framework of choice should be installed as a gem, just like we do for markup languages such as Redcloth etc. Same goes for the template language IMO.
I think there should instead be a GENERATOR to INSTALL the VIEW LAYER (as gems in Gemfile) if required. When run, the user should be given a choice of which template languages to install (ERB as default) and which UJS framework to install (perhaps jQuery as default).
This generator could be run as one of the final steps in the rails app generator.