Well one reason to limit substitution is that clearly the game developers were not thinking that these "vacation substitutions" could last for years on end. In my world, which is 5.5 years old, I know subs that have been in control by one person for probably 4 of those years. He's actually got 5 alts and 15 subs, which he uses to go around and take POIs by influence. He thinks that it's great that you have to waste resources to go blast him off the POIs, which he doesn't care, because they are just subs so he just respawns and does it again. Not sure if that's what the game developers envisioned for this function.