To start with, for those who frequently turn over positions, DMA makes a huge difference vs getting raped by pof. You are right in the cases where price trades fully through your limit price. But even then the price improvement one receives by getting filled earlier than literally being put at the end of the queue several times, makes a significant difference. For anyone who trades and not invests missed fills definitely will have a significant impact on pnl. This is much more important in listed options than stocks. As you probably guessed this problem decreases with increasing liquidity. However, have you gone down the spx500 list to see how many stocks trade above a certain amount of daily turnover? You will be surprised how soon liquidity drops off.
if you know, please show me. i am possibly older than you,kid. i am talking about liquid stock like apple, msft, not thin stock.