As another member previously said, the key difference between the Orkulus and the Argonev/Transencia Starbases is that the former is primarily for offensive purposes, whereas the latter two are defensive. The ability for the Orkulus to move should remain that way, in order to fulfill its role as an offensive unit.
During Entrenchment and later Diplomacy, my greatest fear was finding an upgraded Argonev at an enemy chokepoint. Orkulus starbases were easier to deal with; I often took on fully-upgraded starbases with my fleet, and won (Albeit with heavily casualities). On a defensive position, you can aim for luring the Orkulus away from is repair support, or when its on the attack, the Orkulus often had no support what-so-ever. I'm not saying they're not powerful- they are extremely powerful, but I feel that a fairly good player can take one out.
Now, I don't have much experience against the Advent (They weren't a popular faction after the Illum-nerf), but I certainly dealt with many TEC Argonev starbases. Those things are deadly not because it can charge headlong into a fleet like an Orkulus, but because of the Big Red Button. All too often, my opponent would build one at a chokepoint world and completely shut down my offensive; I'd have to occupy the bordering planets and build up just to find a way around it. When trying to take out the planet, there's not much you can do. You can try taking it headlong, but a supported Argonev is extremely tough to crack, and near-impossible with an Akkan parked next to it (Targeting Uplink lets it take out virtually everything). With the Button, you run the risk of losing everything to the one Starbase. I remember once early in Entrenchment losing and entire fleet of well over 100 ships to one "Boombase", it was terrible.
Of course, you can always park your fleet outside of the Argonev's range, but you still need mass Bombers to take it out. That takes a long while; a defensive world always will have numerous Repair Bays and other defenses. By the time you crack the planet open, the opponent has had plenty of time to bring their fleet back.
You could try to just bypass the planet entirely and move on, but Jump Destabilization takes out your entire fleet's AM and one-third of their HP. Finding a way to bypass and jump away from the Starbase can be both risky and takes time. Then half the time, the next planet has another Starbase or their fleet... :/
From my experiences, Starbases aren't used to actually fight the enemy. They're used as a bulwark; the Starbase's potential damage output is so great into deterring the attacking force from fighting for that area. Trying to deal with it or avoid it just gives the defender more time to move their fleet into either protecting their planets or for pushing more into your own worlds. There's a lot of strategic work in using a Starbase, even if they can't move. I have no problem with TEC and Advent Starbases being unable to move; their purpose is to be so scary and powerful to buy the defender time to either defend or move onwards to attack.
