Well, there's always a demand for somebody who can see both sides.
I never played Colo/XD, but I want to. It's as close to a main-series game as has ever existed for a console, and the Doubles play, limited selection, and much harder level curve seem great for people tired of the same old story. Heck, Gen III was great for story--we got this, Mystery Dungeon Rescue Team where you had to save the world and got run out of town for a stretch, and even in the main series we got our first villains with a goal other than just making cash through exploiting Pokemon. Colo, for its part, is certainly a much different experience both from the main series (limited team selection, Shadow mons) and from other side games (its relative closeness to the main series). Oh, and the music? Pyrite Town is freaking wonderful. Smooth jazz in Pokemon? Sign me up!
I will say that, from my observations (correct me if I'm wrong), having the same, limited selection would really hurt the replay value, although the chaotic randomness of the RNG means the game certainly retains some of it. So I can certainly see why some wouldn't be so happy. (Although let's be fair here: the main series has replay value issues of its own, stemming from that one save file you get. If you want to play through the region again, hope you're willing to dump every mon you care about into the Bank en masse before you delete the entire file.)