You seem to suppose you can only have one dump stat, that's not true !

You can dump Con **if you already dumped less interesting stats and still need more point to min-max your build**. For example if you absolutely need this 16 in Int for your wizard (or to max your knowledge: The Planes if you are playing Pun-pun), with a point-buy of 10 and your GM asked you not to dump any stat lower than 8, you have no other choice than dumping every stat but the Int, Con included.

Yes, it's a pretty niche case, but it can happen !