FYI - I'm fairly certain that CLJS does not use Google Closure because it is "already in java", but rather because of it's maturity and awesome features like DCE.
This and that are not exclusive. There are good reasons to use closure, but would integrating closure and the tooling it requires even have been considered had the cljs compiler been in anything but java?
How many alt.js languages implemented in !java use closure?
How many alt.js languages implemented in !java use closure?