Also true. But if you can prevent the AI from reinforcing to places you are interested in, then the net cost to you is pretty marginal, too.
I guess the core question in this thread might have been: "Is it ever a good idea to kill AI ships just to kill them, or am I wasting my time doing that?" To which my answer would be "unless it's a guard post, command station, special fixed defensive unit (AI eye, fortress, etc), it's going to 'grow back' sooner or later." Starships and other really large ships that are mobile also grow back a lot more slowly, and so do guardians in the main. But going around killing smaller fleet ships is like swatting mosquitoes in a swamp: you're never going to clear out the swamp via that method alone.