I think you're right that there are other angles. But the basic academic approach comes down to inventory management and modeling how the tape influences fair prices. If a market maker is bidding and offering constantly, they will begin to build a larger position long or short depending on what market orders come through and are matched to their limit orders. If they start getting too long or short than they'd like, they can adjust their prices to favor buying or selling as appropriate.
Those little spreads can really add up on a high volume stock and don't forget they get paid for providing liquidity on most all of their orders.