Nonexistent compile times and easier debugging, plus all the other generic reasons you may (or may not) have to choose JavaScript as a programming language.
The same pros that come with using a dynamic language vs. a statically typed one...
It doesn't seem most dynamic languages have huge benefit on large development project. Also, C++ has greater DSL options.
No but they have benefits for small developers. Really depends what you're looking for.
You can hire cheaper developers.
I get the feeling it would cost more to hire developers who can wrap their heads around performance sensitivities in JavaScript and WebGL than it would to hire, say, C++ developers who've read the OGRE manual.
I believe hiring skilled JavaScript game programmer would be harder than hiring skilled C++ game programmer.