If I attack the lead, I am disrupting you for a little. You may spiral out if I get your wing and be unable to shoot back. If you're in the side drones gunning, then you are at a disadvantage for adjusting laterally to my attack angles. If I take you out when somebody else is gunning, I can disrupt your aim if you're in the bombsite, or looking at your map, or any number of things.
On occasion I do (did) focus on picking off a drone first if the other considerations didn't dictate my preferred target. I would do THAT if I felt that the pilot wasn't flying carefully and if I felt wounding a drone would make the lead expand the trailing distance and pop it. Or, if I felt the pilot/gunner was less experienced and gunning from the lead, and then I would use that to make him have to adjust his angles laterally to cope with my attack angles not being directly head-on to his perspective.
Both are perfectly valid, and both can be used, but sometimes one or the other is the better choice given all the variables at play.