Brokerages pass on trades to market makers. They don't promise a price, they just tell you which price is available through market makers, and when you decide to go ahead with a trade, they go on the market, and you hope that the price market makers were advertising earlier is still what they're offering when you come through. Brokerages make a profit by passing on your order and taking a percentage of your total sale, not by making a profit on the spread.
I just find that whenever anyone wants to justify a total waste of resources like some people using microwaves to shave a micro off to nickle and dime the world it always comes down to some guff about liquidity to defend it.