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?
LuaJIT 2 is fast because the Lua world has Mike, who is a genius.
That said, Lua is fucking weird.
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.