Obviously plenty of operators have single battery installations.
Having worked on 737's for nearly 30 years I have never seen personally a single aircraft that has lost all 3 gennys.
We run etops all the time aand fly in excess of 4 hours on a few legs.All single battery.Sure 2 or more of everything is great but given the record of the 737 classic and NG the lack of necessity speaks for itself imho.