What about the FS1 mission where the shield prototypes are being transferred to Ribos? The installation there is clearly guarding the node, its the only point of interest in the area, but the installation is hung back, so as to prevent it from taking direct fire from incoming warships.
Ive always agreed that the nodes seem woefully undefended, but at the same time, look at the sheer amount of punishment warships can soak up. Look at the ships in the King's gambit for example. We were ripping the **** out of them, and on higher difficulties some still escape. This is after running what is it, 3? (i cant remember someone let me know) successive blockades, before reaching Alpha 1 and his god awesome skillz. Wouldn't it then make sense to have a fluid and rapid response from a fleet rather then static defenses? Every credit the GTVA spends on immobile stations is one less they spend on fleet units. Now, since fleet units do bring heavy firepower to bear, and enemy units still slip through, I think mobile units are a better choice, as they can pursue the enemy rather then sit their scratching their asses.
Mix that with the fact that mobile units can then be used as an attack force, and their value goes up exponentially.
Also, look at our canonical sources. The T-V war lasted 14 years, where the situation looked to remain highly fluidic (the terrans were on the doorstep of Vasuda, but unable to push it to victory), I expect any static defenses were wiped out early on or simply weren't funded in favor of mobile defenses and raiding forces, that could exact damage and pull out. Even leviathans, specifically designed for defensive roles, were mobile.
What happens when I build tons of defenses at the Ribos/Antares node, but then conquer Antares? If i used a fleet to defend the Ribos/Antares node then I can simply move the fleet up. If i used stations and heavy fixed defenses, then what are they doing now? Sitting there and chewing away my resources on maintenance. And in a 14 year war, resources are not plentiful. The same can be held true for FS2. Look at the GTVA. Despite the Vasudans miraculous economic recovery, they are still hurting bad, and rebuilding. The same but worse can be said for the Terrans. Mobile defenses canonically are more useful, and there never seem to be enough of them.
Or have we all forgotten how often ships have been 're-prioritized' for other engagements? It comes down to cost.. and well. What I deem to be effectiveness based on the amount of punishment we've seen destroyers soak up.