If there is a good reason to have two banks it is just so you are covered by the dumb stuff. Things you would never think of can happen and drain the whole house bank and you can still start the engine
. The start bank could go and then you could use the house bank. The house bank could go and the lights might work. I have a switch for that purpose.
Out on the water
options are valuable for reasons you can't know until you need them. When things work normal you don't need a PFD
, fire extinguisher, flares or an extra anything.