I would agree that market orders are a bad idea at any meaningful volume (outaide of retail sizes) because of liquidity and routing reasons
Have you read Flash Boys? Dunno if the loop-holes have all been solved, but basically it was possible to front-run orders. There was a regulation requiring brokers to execute an order on the exchange that had the best current price. This rule gave no weight to size. So HFT firms could place a tiny, negative expectancy order on one exchange. Then they could see the result of that trade and cancel/place orders on the next exchange that your broker's matching algorithm was going to hit up before your order got there.
What a broker does has nothing to do with how market orders work. The strategy you're describing also doesn't really work because any respectable broker is sweeping all of the exchanges at once - the regulations considered this possibility and allowed this behavior. Also, many of the liquid symbols have single cent spreads making this strategy impossible.
A market order submitted to a single exchange isn't the same as a "market order" submitted to e.g. Fidelity.com. He's talking about the latter.
It certainly doesn't hurt liquidity. There is, in principle, always someone who wants to trade with me, and the increased volume may make price-discovery a little more accurate.
When I place a limit order, I don't mind at all if someone has managed to front-run my order and sell it to me at the price I set. I got the thing I wanted to buy at the price I wanted to pay.
HFT is only troublesome if you play with fire -- market orders. The price you see may not be the price you get with a market order, as the market can do irrational things or front-run you. That risk is blunted entirely by limit orders. Your order might not fill, but when it does fill, it will only fill at or better than the price you asked.