And includes Mozilla's SpiderMonkey as an alternative to V8.
Really awesome! I guess the reason to do this is basically portability?
Some node cloud providers don't support native code plugins for node. With asm.js, you can compile a native code library for high performance parts of your node app.
It's not so much portability as optimization. Valid asm.js is also valid JavaScript, so it can run in V8 unmodified, but it runs without the asm.js modifications.