Check the comments: how many arguments can be boiled down to "I have to pay for them?"
None of them.
The general criticism on cost is that they're too expensive for what they are. A defensive platform can cost about as much as a fully loaded cruiser or several corvettes. But it has no evasion in an environment where that is still the king of all combat stats. It can't move, so unless this is a key chokepoint the enemy can just fly around it. And it has no chance of emergency disengagement, so is almost always destroyed even if you win the fight.
Defense platforms have cost and upkeep comparable to a similarly situated ship but their only real benefit is that they don't eat up your fleet cap. So in almost any given situation you're better off just building a cruiser or a few corvettes for the same alloys.
If the map were more fluid and modeled after naval war they would have more use. You could build platforms around high-value targets like your anchorages and trade hubs, knowing that those are places the enemy is going to want to hit and your fleet can't be everywhere at once.
But with the FTL change the devs made the (imo entirely absurd) decision to model Stellaris after ground warfare with defined front lines and chokepoints. So players don't choose targets by value, they choose targets based on a trench warfare model of a static front line. By the time that line crumbles, the war is over. So there's no attacking anchorages, shipyards and trade hubs, because by the time they're in range of your ships it's all a moot point. And there's no "my ships can't be everywhere at once" problem, because you can defend the entire front line with one fleet. As a result there's no need for building static defenses around them.
Anyhow, that's what people mean. Platforms are too expensive to be cost effective. Not "people want free defenses."
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