I agree. A backup machine is wonderful when you have to send one in for service.
I have several machines and wish I had room to set up another of them with different color thread. When I need to do a couple pieces with white thread and the rest takes dark. You could just use a different machine instead of rethreading. :? I worked in a dry cleaners doing alterations during college and I had several machines/threads set up that way. It was nice! And a real time saver.