For thick FIFO markets, it is very hard to earn the spread by joining the best bid / offer. You have low queue priority, and will frequently only get filled on your bid when the market goes offered there. In effect, you have still crossed the spread.
Trying to get time priority in a thick FIFO market such as the ES is difficult as many orders are entered well in advance, and cancelled when the inside market moves near if the firm no longer want the trade.
The reason for using limits is certainty of price, as opposed to markets which provide certainty of execution. For immediate entries, aggressively priced limits (at best or at best skewed by 1-2 ticks) is going to be best for ES.
If you are working large orders, you can stack limits in advance in the area you expect the market to turn. For orders less than 400 lots or so, I do not see a point in trying to get filled on limits vs markets or trying to catch part of the spread.
The other reason for this is adverse selection risk. You'll get filled when you are wrong, however on your correct trades the market will frequently trade at your price but without trading through it, assuming you are joining the best bid / offer when you want to trade.
Assume you trade at a low, when the market has stopped going down and has direction has changed to go up. Your signal goes "on" to buy. Instead of paying up, you join the bid and are 990th in line. Four things will happen which will mess up your chances of execution if you are correct:
1) less market orders coming in to hit the bid
2) traders who are trying to go long will be lifting the offer, sometimes in size
3) offers will be pulled as informed traders are no longer sellers
4) genuine interest bids will join yours, and while you are first in queue the bid / ask imbalance will be read by algos who will take the offer and bid the next tick higher
Assuming you are incorrect, you will save a tick on every trade you close at a loss, however you will miss execution on a large portion of your correct trades, potentially rendering your system -EV.
In thick markets, limits are useful for getting filled at a set price when you may not react fast enough to hit market (ie when you expect a spike). Otherwise I am in and out the ES at market (aggressively priced limits actually).