I don't think it would open up as much as you might think it would, TV is used for watching things and not much else plus the ATV has few control inputs. A lot of apps are really just front ends to websites and that would be repeated on ATV app store, why go through all the trouble of supporting that when Apple can just say use Airplay on your iOS device instead to get the video to the ATV. Keep in mind Apple has only sold around 5 million ATV's, which is still pretty small.
Also Apples long rumoured desire to get streaming premium TV channels through the ATV could also be an issue, those TV companies are fiercely picky and all sorts of politics could be going on in the background. They may not want certain apps on the store like Plex for instance.
I think they will do down that path eventually once they get all their ducks in a row.