My software has a fairly large chunk of code which is a gardening expert system. I'm hesitant to call it AI because it's so dumb, but it's along those lines. It knows things like "If you live in Phoenix and it's April, you should wait to plant your lettuce until September".
It's useful to have it on the client, when you're in the garden and there's no connectivity, or when you tweak your settings and don't want to have to download a bunch of "conclusions" from the server. It's useful to have it on the server so that the server can send emails, pre-cache HTML for faster loading pages, etc.
If my server is written in Ruby (which is is right now) then I have to re-implement that entire system twice. That's double the code. It feels gross. And I can't help but feel like most web-apps have code like this that could be shared between the client and the server.