Well a command to exclusively kill a certain type of enemy (Ideally the command just goes away when none are left in the system) would be good.
Think about it, I have a cap of fighters, and there are a bunch of bombers coming at me, with some enemy fighters between us. I don't want my fighters to all shoot at the enemy fighters, then sit around in front of the bombers reloading, I want them to fly right past the fighters, and immediately strike the bombers when in range.
So I think this would work well by just right clicking the bombers with the fighters selected. Once all bombers in the system are dead, the order should go away, and the fighters would return to normal targeting behavior.
To use stand down mode for this, I have to pause the game, stand down the fighters, tell them to go after the bombers, wait until they are all in range of the bombers, then take them off stand down. This is less efficient than just letting them waste the first shot on the fighters, because there's no way they'll all get in range to hit the bombers at the same time.
This situation is more relevant when talking about units with slower firing times, for example, zombards wasting their alpha strike on fighters rather than the missile frigates directly behind them.