One big advantage with Bluetooth speakers is that you can place your Smartphone or Tablet out of sight, especially if used in a public place or for an outdoor party. For those with an existing Hi-Fi or other decent speakers, another option is to get a Bluetooth receiver (example).
A typical iPod speaker dock puts your expensive Apple product in public view and also makes it awkward to use for non-music purposes, e.g. writing an SMS while docked playing music.