Why Not Run Engine with Battery Switch on "Both"

I would like to know why not run engine with battery switch on "both" as recommended on the Maintenance Guide....I do it every time out on the water without a known problem.  Robert Van Meter

Using the switch set to both will cause the batteries on bank 1 and bank 2 to reach a state of equilibrium.  Simply put, if battery 1 is at a discharged state and battery 2 is at a fully charged state, battery 1 will bring down the voltage of battery 2 until they are equal.  Also, if you're using your boat and some equipment/electronics have been on all day with the engine off, it could potentially drain the battery.  If the switch is selected to both, both batteries will be dead and the engine(s) will not start.  If it is selected to 1 or 2 then there is a backup in the event one battery went dead.  There will be no harm to any equipment on board or the engine(s) on the boat.  Electrically speaking it is fine to do this.  Our recommendation is to use 1 or 2 so there will be a back up if necessary.  Charge can be maintained on each bank/battery by switching over halfway through the day of boating to charge the other bank/battery.