I see this mentioned a lot as a theoretical advantage, but I don't really see this happening much. It doesn't take that much to switch between, say, Javascript and Python, or Ruby, or PHP, or whatever. Most web scripting languages are, sure, different than Javascript, but it's not like the difference in switching to SQL or R or CSS or something. I don't really see, in either myself or others I've worked with, much of a bonus from using the same language for client and server side.