I agree with Robert. Always use limit price, even if it's at the bid when buying, this will curve front run activity. Back in the old days, the broker can see your market order and hang on to it for the day and fill it to their advantage at the end of the day, not yours. That's exactly what happened to my 1 market order.
I'm glad it's not the old days anymore. I trade liquid instruments, fills are almost instant, always. It's not relevant how things were back in 1970s(?). Not to trading in 2017 anyway. Sitting non-hidden limit orders are there to be abused by HFT. Many years ago when I was buying the dip, my limit orders were almost magically the low/high of the day - of course I had exactly 0 shares filled at those prices. Draw your own conclusions. Market orders, depending on routing, can leave some HFT players without time to react.
Until the exchange has a system problem or there is news and all the MM withdraw their quotes for a short time. Even in stocks, I'd rather use limit orders through the current market. That 1 in 10,000 time it happens, is painful.
I've been trading for about 12 years now. Never had a broker hold a market order for the whole day and fill me at the worst possible price. Who was your broker and what was the stock? I'm curious if anyone else experienced anything like this. If it was some illiquid stock and your order was a substantial part of daily volume, then it can happen but why would you be doing that in the first place?
Do you remember a trade you did 10 years ago? I can show you a trade that happen last week that filled greater than my limit price. Meaning it was filled at the broker and not the exchange.
Absolutely I'd remember if my broker kept market order for the whole day and fill it at a random time. Price improvement doesn't necessarily mean it was internalized (or filled at the broker as you put it). For example on IEX price improvement happens regularly and even more so on many of the dark pools. That's my experience anyway.