So instead of looking whether the AP count is 0, we'd now look for a special symbol? That doesn't strike me as a gain. Especially considering that the AP counter already does that (you might have noticed that it sometimes changes colour to red, which indicates that the merc can't do a lot of actions in this turn.

• Not done, even though on low (red) APs: One merc has very few APs left (< 10), which makes me move on to the following mercs. After getting done with them, I end my turn and suddenly notice, that my merc with the few APs has suddenly an uncovered flank because one of the following mercs moved somewhere else. In this case, I would be reminded, that the merc is still able to turn, change his stance or whatever else.
• Done, even though on high APs: One merc is a sniper and lays low to keep his lane in view and wait until a foe runs into it to interrupt him. In this case, I could ignore the merc on my turn.
• Automatically done: The only case in which the done mark gets set automatically is on 0 APs. In all other cases it is subjective to the players opinion.