Exactly, IF you specialise your planets like crazy. I don't. I go for self-sufficient planets. That is to say, every planet should have its own production of goods and food and energy, and ideally also generate enough rare Resources to cover the upgrades of the planet. Now, in this style a planet is only worthwhile if it can produce at least one fully upgraded forges Worth of Alloys. If it cannot, then the planet is useless, as it serves no purpuse. It then adds absolutely nothing of Worth.
Yes it would be nice if this was viable, everythign tells me they do not want this because it is very hard to achieve a planet that has net positives in every resoruce. The current design is to have a random inbalance in everything, and good luck dealing with the million fluctuations :/