I don't think it's that big of a deal for various reasons.
1 - the knowledge cost of getting an MK3 Leech Starship in the first place is a heavy deterrent.
2 - you can only reclaim ships you don't have, and the most powerful AI ships (such as Guardians, Starships, and Fortresses, as well as many fleet ships) are immune to reclamation.
3 - the reclaimed ships usually die in a battle later on, so it's not like a permanent force multiplier.
I think someone did a game with leeches recently to see if they were overpowered, and it turned out okay. I think the balance for these is about perfect now. It may look bad on paper, but in practice everything turns out okay.