The SEC values price-time priority because it rewards the most aggressive buyers and sellers and produces fairer prices. It also ensures that market insiders don't ignore retail investors. The SEC has never approved an exchange that did not force orders to interact -- and officials say it's not likely to do so soon. But NASDAQ's market-makers count on these highly profitable in-house trades, on which they collect commissions from both the buyer and seller plus the spread between the buy and sell prices. "If NASDAQ implements price-time priority," says Larry Leibowitz, head of equities for Schwab's trading unit, "the whole market-maker model comes into question -- and all bets are off." Dealers are vital to NASDAQ because they keep trades flowing -- by posting quotes, maintaining an inventory in the stocks they handle, and putting up their own capital as buyer of last resort in disorderly markets. A market that depended solely on ECNs, which don't have such obligations, could break down under stress.
This is a quote from the article and this is why I trade Futures.
Michael B.