Did you know, in the past it was NOT POSSIBLE for you to post a limit order and have it displayed in the order book unless you were an anointed market maker on an exchange?
[edit: this was meant as a reply to gdl not tptacek]
Because automated market making is intensely competitive, the vast majority of that "wealth" was transferred... wait for it... to retail and value investors.
I mean, if a supermarket sells strawberries for cheaper than your local greengrocer, is that "a non-negligible amount of money for a service with questionable value"? It seems to me that the onus there isn't on the supermarket to show that strawberries are worth the price.
People choose to cross the spread and take liquidity. Ask yourself why they might do this (hint: google "execution risk").
Of course if you are time insensitive, you can certainly play games. This basically means running your own HFT strategy, but biased in the direction of the trade you want. You'll also opportunistically take liquidity if the fill you want is in the book.