I remember reading about this on HN a while back and wondering why anyone would want to use it, but now it totally makes sense. I don't want to go back to writing C/C++ but if you could write your app in JS, and then do the tighter "inner loop" parts in LLJS... that sounds really enticing. I'll be following the project closely.
I wasn't convinced of it either at first. You can get around a 5% speed increase with compiled code (because it uses typed arrays and other things which are heavily optimized), and I played around with it for a game, comparing it to emscripten and raw js: https://github.com/jlongster/js-lljs-c-benchmarks/
When I heard of asm.js I immediately realized that I wanted LLJS though. You're absolutely right that it could turn into something which helps you write only parts of your app in asm.js.
Everyone should note, however, that you usually don't need this. Javascript is still highly performant. This is just helpful for things like incredibly complex 3d games, number crunching, etc.
