Currently, reusing code on client and server is far from the first selling point. Companies like Meteor and Groupon are pushing in that direction quite heavily, but for mere mortals it's still limited to reusing a few core libraries like underscore, async or templating.

Reusing experience and knowledge, and reducing the cost of context switching, those are very clear and positive advantages.

On the topic of sharing code, these should be interesting: the 'pipe dream' ( http://keithnorm.com/spainjs-pipedream/ ) and the 'holy grail' ( http://thatconf.chrisjpowers.com/ ).