I generally see Operations Manager as a needed position on a Starship, but not necessarily a command role. Data was a Lieutenant Commander in that position, probably because he was also the second officer and as such required that rank to hold that position's authority aboard ship (i.e. below Commander Riker). We did see in "All Good Things..." that Data had a rank of Lieutenant (j.g.), most likely because of a costuming error; but what if this actually WAS his rank when he signed on, then for whatever reason Picard saw fit to make him second officer and promoted him two ranks accordingly?