Apoorv, there is no big disadvantage in turning off KMS at this point. The boot splash screens will be different, and some combinations of 3D rendering and desktop effects will not work correctly. 3D speed might be better without KMS. However, most bug fixing is now done in the KMS path, so don't expect any non-KMS bugs to get fixed. Please file a new bug if you do not get DRI out of the box (and you have to turn off KMS).
Apoorv, there is no big disadvantage in turning off KMS at this point. The boot splash screens will be different, and some combinations of 3D rendering and desktop effects will not work correctly. 3D speed might be better without KMS. However, most bug fixing is now done in the KMS path, so don't expect any non-KMS bugs to get fixed. Please file a new bug if you do not get DRI out of the box (and you have to turn off KMS).