Using Python and other runtimes is often kind of slow - it's not a huge deal but if you have a lot of this stuff in your config, then vim might start up only like 8 times faster than Eclipse.

The other thing is that the Vim API you use in these other languages is pretty much Vimscript anyway.

For me, Vimscript is basically a normal imperative language in the general neighborhood of perl and shell... I agree it is more awkward to write than Python, particularly with escaping. But it's a decent DSL for configuring text editors, and Python isn't as much.