In what way are they lower in admin-cap use than planets? Do you just mean that they are smaller and therefore you can't build as many districts?
The housing district provides more housing per admin cap than a regular city on a planet, which means you'll get more buildings slots per admin cap investment. As a side note, they also synergize very well with Adaptability (free building slot, -10% housing use).
Every single thing I've heard that Habitats can do, a size 10 planet can do but better, cheaper, and faster. The only argument that can be made in their defense that has at least some merit is the comparatively low admin cap they cost, but since you can ignore admin cap as easily as you can ignore planet habitability, it isn't much of an argument. I do think we're all missing the point though. Habitats are good solely because the powers that be say so. No need to worry about how they function in practice or compare to other APs or anything.
Sure, planets are better in most regards. But the here is the thing: There are only so many of them.
You cannot build planets, but you can build Habitats - and as early as year 60 if you plan for it.
Every extra "planet" you colonize is extra pop growth, which translates into more late game power.
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Fortress Habitats sounds like a nice use, but is it really what a habitat should be used for?
I see it as a secondary use once I have enough (usually around 6-10) refinery habiats.
If you ask me, it Fort Habitats should be a viable specialization. They are somewhat weaker than planets anyway (since those can become fortress worlds for extra bombing reduction). You could argue that they maybe shouldn't provide FTL inhibition, but, i mean, people already call them weak and useless, so...
