I disagree. In a side based fight with vertical separation, it's not that hard to get some separation from the guy you're fighting, and take a poke at a lower enemy engaged with a friendly, the only downside is, you've got to get another reversal on the guy you were originally engaged with. It's a consideration that wouldn't come up in a 1v1.
Wiley.
If all you intend to do is come to the fight at high altitude, shoot the low occupied enemy and immediately skedaddle back to safety, then you're right; learning to duel isn't going to help much. If you ever get into to a situation where a bad guy(s) is planning to shoot at you or you need to maneuver to get the kill, then skills that can be learned and perfected from dueling will greatly help you.
You're 800 out on the six of a better turning plane. He's flying about 200mph, you're about 230. He breaks hard to the right. This is a situation that happens in both the MA furball and a duel in the DA.
The timid player thinks he has no chance of getting the kill without risk. He breaks off the attack and retreats back to safety.
The less timid player that has never dueled or learned any ACM will yank the stick back in a hard turn going for the shot, miss the impossible shot completely, and overshoot. He just bled his superior E state and is doomed because the better turning plane just reversed him is now dead on his six and ready to fire.
The dueling purest sees the impending overshoot and pulls into the vertical and starts a high yo-yo. He's now dead on the enemy's six and ready to fire... BAM. The full time dueler is back in the tower because a P-51 just bounced him. He never saw it coming.
The player well versed in skills learned both from furballing and from dueling, sees the overshoot, checks for any imminent threats before starting the high yo-yo, he sees the P-51, breaks off the attack on the better turning plane and evades or kills the P-51.
To be the most efficient and effective furballer as possible, one needs to know everything.