Some things are not worth doing. like porting hundreds or thousands of old games to linux. but you can run them anyway. (meh!) DX9 will not affect new games being portedas they will probably be going for OGl 4.x anyway. So its a nice thing to have.
And as long as the open drivers are at 20-50% of the hardware speed (taking closed drivers as a reference) game vendors will not give a single fsck about open drivers. They will eventually hope for their fast improvement for a number of reasons. But that might actually be another reason to push AMD to abandon catalyst. Which they wont for the next 5 years at least.
Generally its a nice thing to have possibilities.