The reason there's no real score for ack guns, I believe, reflect's the reason they are there. To help prevent vulching, or other gaminess.
The incentive is to fly; If you make manned acking a cohesive part of the score, then every flak gun in the game is occupied by people who would otherwise be flying.
Plus, unlike aircraft, there is a limited amount of Flak guns, only so many a field, and when those are destroyed, they don't instantly respawn, they are down for at least 15 minutes. IMHO, that's why they aren't scored.