I know the point you're trying to make, and generaly I agree with it. But:
The point of defenses are not to shred attackers so you don't need to invest in a fleet. The point of defenses is to slow down attackers until a relieve fleet arrives and then to help the defending fleets.
Letting the AI bypass defenses won't slow anything down long enough until a relief fleet arrives, really. Your statement is true, but that's done by making defenses destroyable without that much hassle, like LRMs and bombers. They can't shred what they can't touch. So making the AI send in capital ships first to draw the gauss platforms' fire and *then* sending in cobalts so they wouldn't be harrassed would be a perfectly reasonable AI change in that regard, but getting the AI to bypass defenses entirely is a bit much, I think.
Also you can already do that when you micromanage your fleets. But the devs stated repeatedly that they don't want micro to be necessary, so AI improvement in this area needs to be done.
There's a difference between 'necessary' and 'fruitless'.

Right now, generally, it's not necessary as it is. The AI knows to focus fire on defenses, and usually it picks the closer ones. There are other things that could be done to make it a little less necessary (better ability auto targetting, for one thing) while still giving a slight 'edge' to people who can/want to micromanage a bit. The gap between the two right now is fairly large, as evidenced things like the Cobalts charging first and drawing all the Gauss fire and getting shredded in the process, while any player who micromanaged the battle wouldn't send Cobalts against them until they at least had a capital ship targeted. And yes, that gap can and should become narrower, but it should never be eliminated completely.
Making the AI too smart will take any sort of skill out of combat, all it will boil down to is who has more of what ship. I think of micromanaging as sort of being the general of a battle. Without the general, your troops can still fight pretty effectively, but with the general's presense they gain that last bit of organization that they don't have on their own. Same here, without micromanaging fleets should behave intelligently, but not by default use pretty advanced player strategies, otherwise what's to separate a human from AI? Or a skillful human player from a less skilled?
There should always be a slight advantage to letting people micromanage battles, in my opinion.
Edit: Using this same situation to create an example of a slight advantage. If you let your AI engage defenses, they can destroy them smartly with no/few losses (assuming you bring the necessary ships, like Gardas for enemy hangars), and then move on to other targets of the system. If you micromanage, you can guide your fleet around the defenses with no/few losses and engage other targets. Both ways get you past the defenses, but your slight advantage for micromanaging is you have a little more time to destroy the infrastructure before the enemy fleet arrives.