In my prospective, they're just useless. A ship being destroyed every four seconds is largely insignificant for when you're receiving waves of 1001 ships; however, with that said, if you're already at the maximum cap for ship waves, it doesn't matter too much beyond losing much hair from a destroyed III+ ion cannon.
15 ships per minute? That's 6.7 minutes for a small wave of 100, and 66.7 minutes for a wave of 1001 ships.
I don't know about you lot, but more time spent on your planet isn't a good thing, and if you don't have forcefields covering the wormholes [and command station] on that planet, have fun watching them flood elsewhere. I prefer to insta-kill (pun) AI ships before they do any sort of damage to infrastructure.