I'm not sure what you mean - you gain not having to port anything, which sounds pretty great to me.
And for the Ruby C extensions we tried not only did we not have to port them from C to Ruby, they actually ran faster than either the compiled C version or the pure Ruby version because we can inline and optimise between the two languages.
This way it's a lot of work for one small team to write the C interpreter, compared to a still quite a lot of work for every team with a C extension that would need to port it to Python.
And for the Ruby C extensions we tried not only did we not have to port them from C to Ruby, they actually ran faster than either the compiled C version or the pure Ruby version because we can inline and optimise between the two languages.