See, here is the issue...so say, just as an example, you have 2000 series box and you want to restore that backup to say 6000 series appliance. That would NEVER work, as interfaces and everything is different, so 2000 backup could not be restored and thats why you need to have show configuration from old appliance and then copy bits and pieces to new appliance.
Here is what I always do. On old box, from expert mode, run -> clish -s "show configuration" > /var/log/config.txt and then save the file, copy it to a new appliance to same dir and then from clish on new appliance, run -> load configuration /var/log/config.txt and it would error out depending on the line and then you simply fix the line it complained about and do it again. You may have to do this few times (depending on the config), but it does work.
Yes, I agree, its not the optimal way, but best I know of.
Hope that helps.