Well, if Starfleet doesn't pay its personnel anything, or even if it just doesn't pay its personnel in accordance with the amount of brass on their collars, the concept of warrant officers becomes rather meaningless.
All you really need is the doers and the deciders - and you need both in the "experienced" and "inexperienced" variety, so you can't just have one rank scale or career ladder where the top end decides for the bottom end. Starfleet might be egalitarian enough not to pay the deciders more than the doers simply because of silly historical ballast, but the organization might still see the need to give different types of insignia for the top deciders and the top doers, rather than have a Commander give orders to a fellow Commander to get that flag raised on that hill, or for a Master Sergeant to listen to orders from a fellow Master Sergeant on whether the flag needs to be raised, on which hill, when, and at what cost to men and equipment.
Although we have
seen our share of Starfleet doers who hold a commission...