So I guess this is slower than JavaScriptCore and you have to get commercial license to actually embed it. Why would one choose it at all?
"Why? Because V8, SpiderMonkey and JavaScriptCore are all too big and complex. MuJS's focus is on small size, correctness and simplicity."
I'd be interested to see how ES5-compliant MuJS really is. My guess is probably different from what they claim.