It wastes valuable Resources that could have been better spent on making Warships. One Starbase can use up enough Resources that could have been used to build several first class Capital Warships. Also, Starbases can't move- they are fixed defensive points. A determined opponent can eliminate them and even capture them merely by dropping enough Clans on the planet. Build big Warships, not a bunch of weak Starbases.
Warships win battles and carry the fight to the enemy.
Only build Starbases where they can contribute to your overall strategy while keeping in mind race advantages and Strategic position. Next we will cover when to build more Mineral Mines and Defense Posts on a planet. On most planets, the Minerals will be exhausted fairly quickly and the extra structures will only contribute to making the population, both Native and Colonist, unhappy. Also, it takes over 100 Defense Posts before a major benefit is obtained in Planet vs. Ship Combat. For instance, it takes 181 Defense Posts for a planet to have 13 Fighters and 8 Planetary Beam Weapons.