To add on Sonno said, basically this allows you to build extremely resiliant bases past sudden death. Basically you build most of your base behind the reactor, then just throw 5 or 6 turrets out in front of the reactor to protect it. This just needs to get you to sudden death without losing, which isn't hard because you can just keep rebuilding the structures when destroyed. Then once it's sudden death it no longer matters, you can lose all your turrets in front of your reactor but alien won't be able to invade your base.
Edit: I also want to note that even in vanilla tremulous, the best base configurations often put their important structures behind the rc. This tactic is powerful whether you make them invincible or not. The reason? Thee medistation dies in several swipes, the reactor has a lot of health and takes a while to kill. Yes you lose if you lose your reactor, but you probably lose anyway if you lose your last medistation and armory. The reactor makes for a nice shield. In murnatan reactors are even stronger because stackable turrets make it trivial to make your reactor unhoppable and cover the weakness at the top of the reactor
