Okay, I really don't want to continue this conversation, personally, because:
1. I'm really short on time and I think there are bigger issues more deserving of all our attention, honestly.
2. The arbitrariness of any rule is something that often appears in direct proportion to when a player encounters it. IE, rules that were there prior to you first playing the game you tend to take as "just part of the game," whereas later rules are often viewed as "arbitrary." Look back at my post about the alpha prior to starting it, and I mentioned this very thing. Would AIP in AI War have been accepted if it had been a post-1.0 addition? I don't know; it would have been rocky, I think. People like taking all planets, and suddenly robbing them of that changes the whole game. Etc, etc, etc.
Or on the arbitrariness front, why can't you build allied town centers closer together than X in this game? Nobody has complained about that, except to suggest that the distance be increased even further (which it was) to avoid a landscape of uninterrupted cities. And so on and so forth.
We're all a certain amount resistant to change, especially if it wasn't our idea. I'm as guilty of that as anyone. Pepisolo made some excellent suggestions about the difficulty options in the menu, and I shot them down before reconsidering. Same with various suggestions that everyone here has made at various points in time, I'm sure. So I'm not saying this is something that I don't do. But it's something that we all have to kind of look at in ourselves as "is this being resistant to change, or actually on the basis of the idea itself?"
And even more to the point, and one of the reasons I think Kickstarter projects run into so much trouble, is this: ultimately, when a game isn't finished the developers are still trying to refine their idea of what it is going to be. People that buy into the idea early, but then find out their idea and the developers' aren't quite the same, can get disillusioned pretty fast. Especially if money is involved (unlike here). Even if it's a game that they still would enjoy in the end, there's this sense of disappointment that it wasn't the game they first played or first wanted to play. I felt that way about some of the later additions in Minecraft, for instance, so again I'm not immune to this effect.
Anyway, my opinion is that the current rule isn't any more arbitrary than any of the others, neatly solves a problem in a straightforward way, and is something we should leave well enough alone unless there's a compelling gameplay reason to change it. And that meantime we should rip to shreds the other parts of the design that are actually weak from a practical standpoint, not an aesthetic one.