Maybe for you, but I know I will eventually want to try a game vs 2 or 3 AI. Better to have all your bases covered on one board than have to include a card or some such to say "this is what you do if playing with 2 or more AI.
Also, some may want to play two players and one AI.
I see many reasons to do it.
I understand that, and that's why the question used conditional language. However, while I can see putting one or two buildings from the 3+ player count, it seems like there's a pretty high ratio from that player count. That being said, I've only played one game against Starbuck, and perhaps it's not as prevalent with the other captains.