[sciolistse]

We share model validation, internationalisation functions / string parsing and formatting values (language strings, datetime formats, currencies, etc), access rules, configuration settings, views and constants at least.. I've also had a few situations where we've moved client side code to the server, and not having to rewrite from scratch is nice.

None of these are huge issues to solve with a separate language on the server. But for our projects it's been a time saver.