But it still makes no economic sense that you could have multiple planets producing 10 of every single good with all production facilities, and they are still largely useless without money influx from outside (ok, you get a bit money from base pop) via trade with other factions or player activities.
Yet give one of these planets to a neutral faction, and you get massive profits. So there is no economic surplus for you to extract if you own all planets, but there is suddenly more if you don't.
If your internal economy doesn't use credits, you should be able to produce ships/weapons/etc without using credits (with whatever funbucks replace them in internal economy). If it does use credits, it should also be able to generate them (in significant amounts).