Think of it as a replacement for Lua. Something you will be able to include as a scripting subsytem in most other languages.

But LuaJIT 2 is insanely fast, even the Heavy / Fast Ruby isn't even anywhere near its speed. So why would one choose mRuby over Lua?

Heavy Ruby is not designed for speed, and much of its heaviness causes slowness. It's entirely possible/likely that mruby will be in the same ballpark as luajit2 soon after release.

LuaJIT 2 is fast because the Lua world has Mike, who is a genius.

That said, Lua is fucking weird.