It isn't that useless. That 300 + defense does not help much if you have no attack in your defense. Sure, the AI is'nt that good, will go for the rogues first, but with the right units in defense, you can still win fights.
My experience says otherwise (not meant to demean your experience however, just point that mine is different from yours).
On one world, I have had the exact same player attack and plunder me at least five times a week. He has never used anything other than 1 real unit plus 7 Rogues. Due to AI, he doesn't have to. Meanwhile, I have varied my defense probably half a dozen times in the three or so weeks in which we've shared a Neighborhood. For all the good it's done me, I should just use two Spearfighters. My defense on this world is in the mid-200s at present, but it might as well be in the mid-20s; it is utterly and completely ineffective.
On another world, my city defense is roughly 325%. In the first days after a Hood rotation, I will generally be attacked by several players who have more points than I do. Most of the time, even the "1+7" attack is ineffectual, as the damage my army takes is usually 1 or perhaps 2 points per hit, and that gives me time to defeat the Rogues after they've transformed and then the "real" unit. I've gotten several messages from players complimenting me on that level of defense, and almost none will try again. It's not because I'm impervious to attack but rather the cost of attacking and defeating me is so much higher than it is for other players that attackers will just move on to easier targets.
Again, just to be clear: I do NOT blame the player. I blame Inno. It would be an absurdly easy fix to implement and it would make PvP combat a strategic exercise rather than start the attack manually, park your real unit in a corner and then hit Autobattle.