I agree for the most part. I can see needing to switch if you totally run down one battery and the other is partially discharged. You may not want to risk combining them with the battery switch prior to cranking the motor. If you did, the one battery with a partial charge would be trying to charge the dead battery and crank the motor at the same time, which is at best very hard on the battery and at worst will leave you with no ability to start the motor.
The practice of using one battery at a time is old school. The modern thinking is that you should use both at the same time. Your batteries will last longer if you discharge them both a little than one a lot. You should either have a dedicated starting battery, or a monitoring system, or know your usage well enough to not get yourself in trouble, or all of the above.