What you are saying is a very valid concern.
After the flash crash many institutions in the US replaced "true market orders" (where tag 40=1 and has no price) with deep in the money limit orders under the hood, after the CFTC-SEC joint advisory commission raised concerns about the use of market orders in the case of large HFT traders, and concerns on the lack of liquidity that caused market orders that found no limit orders to execute on the other side of the trade, driving the prices of blue chip stocks into the pennies.
We also applaud the CFTC requesting comment regarding whether it is
appropriate to restrict large order execution design that results in
disruptive trading. In particular, we believe there are questions
whether it is ever appropriate to permit large order algorithms that
employ unlimited use of market orders or that permit executions at
prices which are a dramatic percentage below the present market price
without a pause for human review
So although you still see a market order on the front end, it is transformed to a very aggressive limit in the back end. However, doing this change manually, by selling at price 0 or buying at 9999 may backfire since it may trigger fat finger checks and prevent your order from reaching the market. For example BATS Exchange rejects orders that are priced too aggressively and don't comply with the range of valid prices.
If you want your trade to execute right now and you are willing to take slippage in order to get fast execution, sending a market order is still the best alternative.