Dead asteroids don't have any resource asteroids, but the Akkan can still colonize them. You just don't get any bonus from doing so. If the planet has 2 asteroids, then that's the maximum the Akkan can give you, simple as that.
Anyways, just running through the math, the Progen with equal level colonize would still offer more cost savings in most cases. The only case where the Akkan would be better is if you only purchase only one planet upgrade. If two or more planet upgrades are purchased, the Advent still get better savings. After that analysis, it seems pretty cut and dry to me that the Akkan should get 1/2/3 extractors.