Planets can and do launch drones themselves. Unfortunately, they will launch ALL drones regardless of type or the size of the threat.
If you design bases to use units you must include both the SY to build them and the appropriate launchers. In fact, include ALL types of unit launchers on a base if you give it cargo since you cannot control what the AI will build with those SYs. The AI is not smart enough to find units in the cargo of bases, or to load the appropriate units onto bases with launchers. I discovered this by designing BSYs with cargo space. Any units built by a BSY with cargo will just sit in the cargo regardless of what the AI needs. Only when the cargo over-flows and drops units onto a planet will the AI find and use them.
At least with all the necessary launchers the base will launch what it has if/when there is an attack.