Also that's going to potentially add a ton of work for people developing libraries. With a single engine behind it, the coding efforts can be focussed on writing and profiling something to be fast on v8. If you get replaceable engines, now developers have to either: 1) Ignore all but V8 (or spidermonkey, or chakra etc.) 2) Create different versions of their libraries for different engines. ( 3ish) write multiple paths in their code depending on the target engine)

I'm not sure exactly where I sit on this one. In many regards I think I like it, allowing developers to use the right engine for the right job, for example. Each engine has its own strengths and it may be that v8 isn't the engine for you / your workload. It's just this could hurt as much as it helps.