Most catamarans do not have the same hull speed limitation. Their sail power is translated into speed. Putting drag on the system by charging the batteries slows the boat, which is not usually acceptable to the cat owner in most cases -- that's part of why they own a catamaran -- speed.
BerTX, you are correct in your comments, but I would offer another perspective. Lots of multihull owners buy for stability, square footage per hull length and shallow draft rather than speed per se, (although they don't mind the speed) and I bet those owners would gladly trade an 8% speed penalty for being able to have electrical power underway without a generator running ruining all the ambiance. I love the idea of having an electric motor driven prop that can motivate or charge a battery source, thereby negating the need for constant generator running.
On a side note, no one would want a pure electric sailboat more than me, but as you correctly point out, weight is critical in multihull boats, and any weight-acceptable battery system would likely only be adequate for short distance propulsion or negating generator usage.