Objectives can be defended/attacked, but as such they are known possibles areas of engagement and as such defensive constructs near possible AOs certainly make sense.
This is actually one of the biggest problems I have with the way the map is currently run and generated, which I feel like ties in with defenses.
I
hate the move toward front lines, chokepoints and trench warfare. I can't understand the decision aside from it simply being easier from a game design standpoint.
Warfare should instead be modeled as you describe, around objective points. It should be a naval, island-hopping style. When I'm at war I should have to consider and prioritize my targets based on their strategic, economic and tactical value. That, in turn, creates a wide variety of systems for me to have to attack and defend.
Defense platforms would have more use here simply because it would encourage players to split their forces. Not knowing where the enemy might attack encourages me to hold some ships in reserve, and having to choose from among several options encourages me to hit several at the same time. Throw in the range/supply/logistics issues that players have been requesting for over 2 1/2 years and you might actually have an interesting war game.
Front lines, on the other hand, encourage consolidation. The whole war is mostly won or lost in a single engagement; you either break through or you don't. Choke points multiply that even more.
Defense platforms are useless in that environment because, as others have pointed out, they simply melt away in the face of a concentrated fleet. While they would be useful in a mixed tactics environment with ships spread out across the map, Stellaris has been built to reward ever greater doomstacking.