Other than the publicity stunt, which it most definitely is when compared to the cost of CGI, there is one thing we can't believably CGI away: microgravity.
When actors are in harnesses to simulate microgravity, you can generally tell. Also, their hair will still obey the laws of gravity, as will any object they interact with. That doesn't matter much when it's one small shot in a movie, but it does very much matter if the movie spends a significant time in microgravity and is aiming for some degree of realism.
The best way to inprove the realism is by building a set on a plane which goes in freefall (better known as the Vomit Comet. It creates believable microgravity for the actors and their surroundings (e.g. Apollo 13 used it to great effect), but a plane cannot freefall very long and you end up with short scenes (or frequent cuts).
Apollo 13 behind the scenes footage for the microgravity scenes.
If you compare Apollo 13 to Gravity, Apollo 13 feels a lot more believable from a physics point of view.
Shooting a movie in space would allow for significantly longer takes of a microgravity environment, since you'd actually be in an actual microgravity environment.
That being said, I very much doubt it's going to be cost-effective, and thus we get back to the already made "publicity stunt" argument.