I daresay you seem to have wrong impression. Having Rails app for typical web stuff and Eventmachine (em-synchrony) + faye stuff for realtime things work pretty well in actual usage. I can wrap DB calls, memcache/redis, SMTP connections etc inside my event loop and can achieve near same performance as a Node app.
The alternative of using node.js for everything sounds good but I am not really sure, how good node and its web frameworks are for building typical CRUD applications. Rails really solves building database backed applications problem pretty well.
I receive some kind of request in a Rails controller. I need to send, as a consequence, a notification via the web socket. How does it get from point A to point B? In Erlang, it's all likely to be quite contained and fast, and not involve writing anything to external queueing software, and to boot, it's all going to happen in the same unix process, rather than having one process that's the Rails server, and another that's the eventmachine server.
I don't know Node.js and how you'd handle sending messages from point A to point B, but I'd be interested in hearing some opinions.
The alternative of using node.js for everything sounds good but I am not really sure, how good node and its web frameworks are for building typical CRUD applications. Rails really solves building database backed applications problem pretty well.