Note though that your own Lasers, Particle Beams, etc, will be useless in atmosphere.
There are two other things Girlinhat left out, first PDCs get 10 armor levels for "free" (they are deep underground) although that doesn't matter as much in the later game when you have the industry to spend more time armoring them.
The other big draw they have is that they are hard to see. A large orbital station will easily be seen by the enemy, even with no EM or thermal emissions, the Active sensors will pick them up.
PDCs are much, much harder to spot, in fact I'm not entirely sure how you can spot them. Obviously if one has an Active sensor and turns it on your enemy can see it. And I think that launching missiles or shooting mesons also gives away your permission. However I think that other than that ground troops are the only things that can spot PDCs.