i think it depends on one's trading style, and the market one trades
i trade mostly YM and almost all of my entries are limit orders.
there are a few breakout type setups i use where i will use a buy stop to enter long, or a sell stop to enter short (thus, at market) but 90% of trade entries are limit order
look at this way
if you use a market order on both sides, you lose the spread (some people think you pay it twice. that's false. buy the ask, sell the bid = one spread). if you enter with a limit order, and sell with a market order, you do not pay the spread. if you enter limit and exit limit, you make the spread
in the long run, that makes a difference. again, depends on your style and market
for a scalper, entering limit makes a BIG difference