He's the oldest mutant on the council with millenias worth of knowledge about the world and mutantkind. He's skillful and has powerful tech (celestial) that can be useful to them. Also they know what can happen if they dont have him on their side. AoA and Cable's future. Apocalypse would never be a passive civilian living on Krakoa so giving him a seat at the table wasnt a bad idea. It makes sense