Your method will stall responses for server shutdown + server startup time, which for Ruby/Python apps is usually measured in tens of seconds, and for other web servers can be much worse. Hot code reloading lets you avoid any downtime at all, and with it usually being built into the framework/language specific server you get the functionality "for free".

Zdd (the project I linked to) is all about spawning a new process in parallel. All the advantages of your approach (switch to an entirely different language? Who cares) but without the stalls.

Zdd also lets you keep the old process alive through the duration of the deploy (and after), and with a little work could let you switch back in the event of a bad deploy without having to start the old version up again.