My experience in user interface coding (though not really my favorite area) has been that event handling is pretty much code and ignore, until you want to do some big nasty thing, at which point you have to implement a callable/runnable interface (in java), queue things up and do some synchronization. This seems to be less of an issue in the browser, though I haven't looked at the implementation details.

I do agree that having some kind of multi-threaded greenlet/fiber dispatcher would be more efficient and handle corner cases better. I just don't think your average developer would notice a huge difference.