Cole was the man who showed Phoebe what it meant to love someone. He risked a lot to be with her even though she was a witch who happened to be a member of the Warren line of witches. Phoebe risked a lot to be with him because he was a demon who was also half-human.

I got the impression that they had her settling for Coop because it was the final season. Possibly, if Julian McMahon had still been in the show at that time, they'd have had Cole and Phoebe finally reuniting at the end, instead.