So, I've been putting the beta through it's paces, mostly with advent. The starbases are pretty cheap and fast to build basic, but more costly if you wanna fully upgrade them. So far I've found that my favorite use by far for my starbases is to act as trade ports in non-planet systems to fill in the holes and extend my contiguous trade route to epic lengths. The defensive capabilities are somewhat disapointing given that it's guns have less range than a standard defense turret, and although the longer range spells can put some hurt on, it still would not be too hard to simply fly around.
What have my fellow beta testers been using their starbases for?