would increase the number of enslavers, that`s for certain. It would also make it harder for attackers of UWs, conceivably. A defender, who already has borders, instead enslaves you, then punishes you the maximum amount, after which he frees you, then does it again when you continue to try to fight. All this while, having enough borders so that if he gets hit in return, he can immediately continue fighting, whereas if he had to plunder three planets to be able to attack, if you hit him back, and take colonies, he may need to stop and explore new borders if he doesn`t have any more, or risk losing his precious planets.
I`m not sure how big an effect that would have, but at the very least, it would save attackers up to 15 turns, and make it more of a pain for those trying to build up.