I think that it's relatively common for people to put Active Sensor PDCs on the moon, as they are the most likely to be attacked and the least sensitive to range.
I generally don't use PDCs that much at all though. I prefer Orbital Bases, not so much for the beam weapons (although that's not true anymore in my "no missiles" game) but for their mobility. It's nice to be able to tow them around as the front lines change, and they aren't all that expensive to refit given their lack of engines, so once you build an Orbital Base they can remain useful in combat for a long time.