Nordhavn did a nice project with the diesel-electric propulsion. For the owner of the boat, he like the security knowing that he could lose one, or both of his main engines, and still run off his generator, 3 prime movers to keep the electric motors turning (system would also run off the battery bank for about 4 hours, the owner said it was his favorite trick to pull up to the docks silent)
On larger sail boat, running both a main engine and a genarator, couldnt this type system be used, to give you a fail safe if one engine fails?
Yes, it could. The Nordhavn of course has room for a larger capacity of batteries though. That is a good example.
This type of configuration allows for the best of both worlds, though I've never actually been on a sailboat that was a hybrid like that.