Another reason with Rust at least. Rusthon can make the same memory guarantees as Rust.
Personally I wish this was done with Ruby.. But then again I'm not writing it myself, so I can't really complain.
A random last thought, it would be really something if Rust could formally prove its memory safety and then push that whole system of lifetimes and borrow checks into LLVM or a similar abstraction on top of LLVM. Because Rust as a compiler target is very interesting but it wasn't really built for that.
[0] https://github.com/dropbox/pyston/graphs/commit-activity
Pypy is pushing towards full 3.4 slowly, which is good because I want some of that Juicy STM support in my web apps.
Python(2) code/libraries will mainly survive on PyPy/Pyston going forward. With stuff like Nim and Rusthon, I think something like those will carry the torch forward after Python(2), rather than Python3.