It's strategy, you have to react to the situation, the PLD can exploit the weakness of a SMN by taking the Fester hit of the target you had intended to hit. There is nothing wrong with that. As you use Fester since you're hitting the PLD and they don't have DoTs, they take less dmg.

Same with Piercing Talon, should a PLD take 2k dmg because the target they are covering is at 10% HP? No, it should only be the worth of who the target has changed to.

Truth is you as the SMN you could just use Energy Drain rather than Fester if you know the PLD is covering the target or if you are in a situation in which you think the PLD could be covering your target use Energy Drain. What's more important? Losing some DPS because you have to use Energy Drain rather than Fester or losing a stack Attunement because you chose to use Fester. The options are there for a reason, you aren't disabled from gaining attunement if you react to the situation properly.