Question: Does anyone use a shell written in Javascript, Ruby, Clojure, etc as their everyday unix shell, instead of bash, zsh, etc? Is there any reason not to?

Answer: 
Well, you could use `ipython` as a Python shell. It works, as I reported [here](https://unix.stackexchange.com/questions/81161/change-the-default-shell-to-python). 

But you might want to read [this answer on](https://stackoverflow.com/a/3640403/1071869) SO as for why Python, Ruby etc... won't make good languages for a shell.
A colleague of mine used to have `emacs` as it's default shell. :)

Other people like `screen` as a login shell for remote systems.

You can even use plain `irb` as a shell. (There is a [module](https://stackoverflow.com/questions/166347/how-do-i-use-ruby-for-shell-scripting) for adding the basic shell commands as ruby methods.)