I'm thinking that with well insulated tanks and good stratification, extra gallons won't hurt much. You wouldn't necessarily have to heat all the water fully - as long as the hot water is at the top, and the loads are taken off the top, extra cooler water on the bottom of storage won't affect loading/operation?
You are right And you can always fill you boiler full.
I have a smal tank to my boiler and have to wait to the tanks go down in temp before I can fill my boiler full.
Bigger tanks make the system more flexible. You can make a fire when you have the time..