For example, from the archlinux wiki:
As has been said, recovering should work when reinstalling mesa and libgl.Warning: Using the installer from ati.com/amd.com is NOT recommended for inexperienced users! It may cause file conflicts and X failures. You must be familiar with booting to the command-line if you wish to attempt this.
But how exactly should AMD do it better? I mean, they could back up the files they overwrite, but then they would maybe restore old files to a way newer mesa/libgl release that has been happening in the meantime...
And I wonder: When you use the ati installer and then update mesa/libgl, won't the package manager overwrite files belonging to fglrx and break it?