Crucially (at least on my device,) there's no granularity to this opt in - I cannot choose to backup my application data but not my WiFi passwords. Now my options are to either accept Google's storage of my network passwords or set up and maintain my own backup system.
That assumes your application data is less important than your WiFi passwords. I don't think a distinction at that level is useful. To be useful, it would have to be per application. I don't care if some game status is synced, but I surely care if my email credentials are (although caring may just means I'm aware, it doesn't necessarily mean I would change it).
