Also note that Emergency Build allows you to build these Spaceports in a mere two turns, at no cost if the planet in question has less than 10 slots.
The main use of Natural Merchants is to be safe from losing your spaceports in a given system, and even then it is only useful if you have a low number of systems (for example, in NGC5, four Empires had a single system at their disposal, with an insane number of good planets. But if these Empires forgot to build several spaceports, it would have been very easy to shut down their whole planetary production with a single offensive).
If you have a big Empire, you will either not feel the loss of a single system, or you will suffer from massive unhappiness if you cannot defend yourself, resulting eventually in the dreaded riots. Once 95% of your planets are not producing anything, being safe from "spaceport loss" is of little use to your collapsing Empire.
