ZPGs are always worth it. Saving 20 AIP is big, and saving yourself the headache of needing to protect the randomly placed ZPG from CPAs and Exowaves is huge. Plus, you can capture the second one anyway, and build MOAR TURRETS.
Mass Drivers are fairly cheap and are the quickest way to deal with enemy Starships. I always build one on my HW if I can.
The Radar Jammer II and the Armor Suppressor are big-Big-BIG if you are trying to build a serious defensive stronghold out of a system.
Ion Cannons are never worth it.
But, yes, the Zenith Traitor can easily give the AI tools, too. The most annoying instance I can remember was one game when it sold a Black Hole Generator to the AI in a system that my entire fleet was using to return to my homeworld through, so I could defend against a wave. The wave came, but the fleet hadn't arrived... and still didn't arrive... and my turrets were down and WHERE was my fleet?! Three systems away, trapped in an AIP-free BHG.
And, of course, the badly timed OS Raid Engines, or poorly placed Superfortresses, or an extra FF Mk V over the AI Command Station, that can ruin a day.