The problem is the 'freeze' time is only like, fifteen seconds, so you'd need four or five of the damn things, or impeccable timing.
That's only true if you're looking to get a complete lockdown. Kosturas are VERY useful even without that; my fleet attacks a well-defended system and engages an even-match enemy fleet. Normally this'd be risky, with a lot of losses, but if the planet is hit by two or three kostura strikes spread over the course of this fight, things swing WAY in my favor. For those short periods his ships aren't moving or firing, his capships aren't activating their annoying specials... only the fighters are still doing anything during that period. While his ships are stunned, I'm focus-firing his capships to death, and if he tries to run, another shell will stall his ships again; if I decide to withdraw a wounded capship during one of the stun periods, he can't do anything to stop it either. Plus the damage is nice, of course; it might not do much to each individual target, but it hits EVERYTHING, making it far easier to quickly wipe out the dinky frigate helpers with AoE attacks (desolator missile swarm!) and strike craft.
They're also great for a general annoyance, which is why I put at least half of mine on auto-targeting and only save a few for direct assaults. The enemy's trying to shuffle his fleets around, run a trade ship network, etc., and suddenly random back-line systems will be frozen for short periods, incapable of doing anything? And if he happens to have just started construction on something when the shell hits, it's blown up and has to be rebuilt (at full cost)? Over time that adds up into a big economic advantage.
