The reason behind "yet another template language" is that we compile down to both Python and JavaScript. Allowing us to render templates both on the server-side and client-side.
Sharing templates between these two gets very important when you develop real-time web sites.
(I'm one of the developers behind Planet Framework)
Ah. That makes sense. Both Jinja2 and Mako are very much tied to Python. (Though compiling a subset of Jinja2 AST's to JavaScript would be an interesting exercise...)