I am not sure if you have seen it happen, but whenever I try something similar against the AI to what you have in the pic, the AI would respond with something I call "dumbell formation". It would throw all it's melee and cavalry at my flanks (huge blob on the poor hoplites or whatever I have there), while the AI's missiles would focus on the pikes
It's a mixed bag, sometimes the AI does try frontal assault, such as the Iudaea battle in Egypt AAR (Part 8). But yes, what you describe is also very common, as seen in quite a few other AAR battles. My flank Galatians, Nubian spears, and Thorax swords often get quite a workout. Main reason why I'm thinking of reducing the number of pikes and replacing with more melee infantry for the flanks. Also will probably drop one archer unit as well. I don't really mind the "dumbbell". Splits the enemy army, often leaves those missile troops exposed. Many times I can wheel a couple of phalanxes to the right/left and create a V-shaped vise surrounding the enemy blob. Sure, it breaks the integrity of the overall pike line...but the enemy center is empty as well, except for now-exposed missileers in the second echelon.

Never occurred to me about the ballistae being "aggression bait"...but yeah, I think you're right. This actually strikes me as a plus for a pike-centric army...one wants the AI to charge.