To get the $5,000 discount you are assuming all the LIMIT orders get filled and that any MARKET orders would have been filled at an inferior price. This would not be the case. In reality you would end up with a lot of LIMIT orders that need modification and perhaps filled at a much worst price than if a MARKET order was used originally. And if your stubborn enough maybe even have the order go unfilled. Do you have any real life examples of someone getting into trouble with an Interactive Broker MARKET order, any ET threads on this?
With logic like that, it's tough to compete Over the years many here have posted that their spread orders on the SPX tend to be filled near the midpoint. Given that SPX option spreads are very wide, for an active trader, just placing a spread order with get you more than that "discount". Your advice is tantamount to saying, never place a spread order because you might miss a position. But my favorite reason for doing a market order is your previous statement that: Another big plus is that you get the trade over with and can do something else. Yep, that sounds like a real dedicated money making endeavor.
"Another big plus is that you get the trade over with and can do something else." And chasing a LIMIT order until it gets filled is a money losing endeavor and time waster. I don't consider watching the monitor for an order to get filled very productive.