Sounds like maybe people just didn't want to use those forms of defense, they wanted to be able to build the planetary defenses from the planet itself. I know in one of my games the only defenses I have are the orbital arrays. It all depends on what you decide to research, mine deployment and orbital defenses are easy to research to throw up, but the cruiser/battlecruiser transforms take a coupla research items each, since first you have to research the ship itself, then the tech to transform it (don't remember what tier those techs are, but if it's not tier 1 for the ships and the transforms, thats even longer before you can get em, whereas mine deployment and orbital defense i believe are both tier 1). As for minefields, I have no idea what kind of attack they have for how effective they are.