Well, I guess that's another way I didn't understand things when I was last playing. I just kinda bumbled along and hoped the numbers turned out good.
The term we use around here is "role-playing".
I guess I should actually contribute to the thread, though. Basically, ships and units do two things:
Ships provide PPV (Planetary Protection Value) based on their weapon power - which means ships with a lot of cheap weapons (plasma carronades) provide a lot of PPV even if they are not combat-effective. The population of your larger colonies (over 10 million, excluding your capital) require a certain PPV value summed over all the ships
in system to feel safe and secure (meaning you can put a fleet at Earth and your pops on Mars and Pluto will feel safe at home). If the PPV requirement is not met your citizens will feel unsafe and unrest will go up.
Ground units do not provide PPV but do provide a suppression value that can reduce unrest. I'm not sure, but I believe the formula is SQRT(size) * number of units for each type of ground unit present on the planet, which means the best suppression units per ton/BP are INF with PWL. These can reduce unrest on a planet, but since you can have multiple colonies gaining unrest from PPV it is usually going to be better to just station some ships in-system instead of building a new ground formation every time a colony hits 10 million. Of course you want garrisons and such in most colonies anyways, so it may be a moot point depending on the exact values involved.
If you want to cheese PPV, just build a "ship" that is nothing but plasma cannons without even an engine or a fire control and plop it down in-system. Call it a "system defense platform" if it makes you feel better about the cruel deception you are committing against your citizens.
(Disclaimer: I might be wrong, and it may be that the population demands that you have at least one fire control, etc. before they'll accept your "ship" as a real defense. If so, add whatever is needed since it's probably a good idea to at least make the ship able to fire its weapons just in case, the same general principle applies though - load a cheap hull with a lot of plasma guns and call it whatever makes the people happy.)