The AI has to be effective with them against fighters. A lot of that is only really effective against fighters when used by the player in specific ways fairly certain.
This 100%
Plasma cannons are definitely one of the weapons where the player can easily identify situations where they are very good at killing fighters (the fighters are grouped and flying directly towards the ship). If fighters are not flying directly at the ship, the combination of low shot speed and slow turning rate make it very difficult to hit fighters meaning lots of missed/wasted shots. In addition, firing a PC costs A LOT of flux so hitting only one fighter is not good enough, you need to hit a whole group.
Making PC good against fighters is practical for a human, but the AI firing PC at fighters would almost certainly give very bad results most of the time. As far as I can tell, the AI behavior is 'fire at targets in range, in order of priority, unless the flux is too high' except for strike weapons.
I don't think the AI is anywhere near making those kinds of decisions well, and my guess is that it would require an impractical amount of computational resources to make that sort of decision in real time for every weapon on every ship.