For what it's worth:
TL;DR
A compromise – A dominion needs to have a significant number posts written by a faction. Pick a hard number and use that, written by a minimum number of unique writers from that faction, regardless of anyone else in the thread.
Longer Version.
I was leaning yes because some people will always rush things for the sake of the map, because that’s what they find fun, everyone has different types of fun. For me personally, I am now saying I’m not in favour of this, at present I have nobody multi factioned either, so it’s not about personal OOC play for me at present.
I think it restricts creativity too much with little gained. So someone gets another square on the map, nice OOC but IC it’s about story, for me anyway. As an example: I can completely see [member="Taiden Keth"] for example my prideful Echani, assisting a faction with a dominion for his homeworld, if he thought that faction were doing the right thing. But I have to miss out on that because of some arbitrary rule somewhere?
If we are really talking about the meta, if someone wants that square, they are going to invade it and if the faction that took it has nobody to defend it, that’s game over for them. Also someone can just make an alt to boost a factions numbers, so it doesn't make any odds.