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.
The two advantages of using Python for me are being able to use utilities I've already written, and the comprehensive standard library.
For example, I wrote a macro to split long Python function calls into multiple lines, and I could access a parsing function that I already wrote, along with Python's own compile() to detect when I've parsed a complete expression (yes, this one is Python-specific).
I also wrote a macro to submit the current file to a private pastebin, and AFAIK you can't make web requests with Vimscript.
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.