I'd rather have Lua to be honest... But JS is better than AppleScript in this regard. While easy to understand, it wasn't suited for large projects and debugging was kinda nasty.
The Open Scripting Architecture (OSA) used to support additional languages instead of just AppleScript, I recall that Perl was one of them in the past. Unfortunately, this changed at point. I'm hoping that Javascript support points to them re-opening this.
OSA does not support things, it's the underlying platform hook. AFAIK third parties have always been free to implement OSA bridging. In fact one bloke did exactly that[0] when JfA was first announced.
Yes, you could already do Lua (see LuaCocoa) and other languages like Python (PyObjC), Ruby (RubyCocoa/MacRuby), and even JavaScript (JSCocoa) via ScriptingBridge way back from 10.5.
Ubiquity should not be an excuse for the promotion of what is, by all objective measures, a rather poor programming language.
This is especially true when it's an environment where there are numerous other languages available, and they're all so much better than JavaScript. Lua, Python, Perl, Ruby and even Tcl would all be better choices.
It's mildly excusable when it comes to web browsers, because JavaScript really is the only viable option. But that just isn't the case with a full-featured environment like OS X.
