"My name is Ozymandias, king of kings: / Look on my works, ye Mighty, and despair!"
Habeo in animo vivere in perpetuum aut mori dum conor.
If these neutral tribes are coming into play in any way at all, one of the players is doing something very, very wrong. If you're subduing a neutral tribe, either you've already won and are just prolonging the game, or you're about to suffer a loss.
There are cases where they're useful. It's unreasonable to say that they should never be important. I think in the cast majority of games they should be unimportant, but there's no reason to talk in absolutes.