Whil i havent ever used a boat with this setup myself i have read
accounts of people who have, and i believe the advantage has not much
to do with cruising or motoring... the batteries give you enough juic
to get out of your mooring only, not for much more, but things get
interesting when you set these things on 'hybrid mode' (might be called
something else, cant remember)...

In this kind of mode, you set the motor to a certain speed, say 5
knots. When the boat slows down, the motor kicks in instantly. when
it goes above this, the motor goes into generate charge mode. This
might sound pretty useless, but imagine yourself in medium to big
seas... boat sails through a trough, up a wave face and starts to slow
down (sails emptying)... motor starts and *automatically* powers you
up the wave face. you reach the crest, sails fill, you surf down the
other side, motors charge. Its my understanding that this sort of
sailing gets a lot more pleasant with a motor like this whereas people
with diesel only might just be motorsailing constantly. From what i
read i think the batteries would support a whole day of sailing like
this pretty easy.