Quote from chvid:
I think people have more trust in limit orders and value their transparency - they fear that they may be cheated in some situations with a market order.
This may be irrational.
I certainly found that my system gets adequate prices and fills with market orders - having said that - my system may be more tolerant than others.
It really depends on the stocks you trade. For liquid stocks you probably be fine with market orders, but for stocks that trade say less that 5M shares per day, it's only the question of time when you replace market orders by marketable limit orders in your ATS.