If you intend to write plugins you definitively should read the nice article "Writing Vim Plugins", by Steve Losh; not only for deciding if you will stick with VimL or not, but for the best practice advices.
It also contains a small discussion about Scripting Vim with Other Languages:
First, using another language will requires your plugin’s users to use
a version of Vim compiled with support for that version. In this day
and age it’s usually not a problem, but if you want your plugin to run
everywhere then it’s not an option.
Using another language adds overhead. You need to not only learn
Vimscript but also the interface between Vim and the language. For
small plugins this can add more complexity to the project than it
saves, but for larger plugins it can pay for itself. It’s up to you to
decide whether it’s worth it.
Finally, using another language does not entirely insulate you from
the eccentricities of Vimscript. You still need to learn how to do
most things in Vimscript — using another language simply lets you wrap
most of this up more neatly than you otherwise could.
My experience is that even when a non-VimL plugin is better, I end up switching to a pure VimL alternative later, mainly because of portability. Vim runs on virtually any system (even ugly and old legacy systems), and the overhead of setting up the dependencies or temporary disabling that plugin is not worth (especially if you keep forgetting that you disabled it and trying to use its mappings/commands).
Even when it is easier to setup the dependencies you can hit some problems (e.g.: some python-based plugins doesn't works 100% when they are sourced from shared folders on Virtual Machines). That is why the few plugins I wrote use VimL only.