-- Electric power storage (batteries) seems like a good idea so that the generator can be run at either max power or shut down, but that presumes there aren't any smart load-balancing alternatives, particularly given that there are other electrical demands and electricity sources on board.
-- At present, a large battery bank contains a lot less power than an equivalent weight of diesel fuel. Using this measure, battery technology has a long way to go.
I think the present competitors are 2 diesel engines+saildrives versus a single diesel generator+2 electric motors+prop units. If they use close to the same amount of diesel fuel and produce similar propulsion, and the electric system weighs less, takes less space, and is easier to maintain, I suspect we will see more diesel electric boats. They may be less reliable than a 2-diesel system, but when that difference is actually quantified, it may be insignificant to most cruisers, particularly if diesel-electric maintenance is simpler/cheaper. After all, it is supposed to be the auxiliary propulsion system.