We could consider an increase in build time and cost of pds of all factions and all tech levels(having in mind the arty and the anti air as well) by some percent, such that it would be devastating to the economy of the player attempting to build them, if he/she doesn't truly have map control and a fairly earned better economy.
In exchange, the pd should have more horsepower.
I think, that the tactical benefit, of space control with static defenses, should only be viable to those players, that by their skill have achieved a largest control of the mass points, and are thus "entitled" to build a structure that is more deadly with less sweat.
You can also make them explode when they die, if they 're not closer than 2 km from 3 of your factories.
The same applies to walls. Right now they are very cheap to build, though they do have unjustified amounts of hp.
The point of the above being, not to be able to use offensively(at least easily) defensive toys.