It depends on where the emphasis of your system and your style is shifted. What do you earn: high probability or high RR. If your system is trending (high RR and low %+), then it is advisable to consider additional purchases. If the system is sharpened by probability (low RR and high %+), then the use of additional purchases is harmful. ES is not a trending tool.
I suppose it depends on who you ask and on what time frame.

Good comment, though. I do agree that most of the time - adding to a winner may not be the best strategy on ES. But it really boils down to targets and strategies.
If you bought near the LOD this morning you could effectively add to your position when the market broke higher. As an example...