Inojin was necessary to the plot

Based on my understanding, the attack on Inojin could have been part of a strategy to disrupt the defense of the Leaf (plot wise) . Ino's connection to Inojin means that an attack on him could potentially disrupt the telepathic network. It's possible that Inojin isn't actually dead and may have a secret trump card up his sleeve. The fact that he touched the shinju claw grimes foot could be significant too, but this event might mark the beginning of the Leaf's downfall.