Regardless if its a big or small issue ...
At the end of the day, if its not suppose to let light leak into the camera and affects the metering then its a flaw in the product. Im happy that canon is willing to repair affected units but its still a pain.
If you paid good money for the product it should be as perfect as it can be. Even if this has no effect on your photography its always a positive to know that Canon has released a product that does everything it is suppose to with peace of mind. I paid good money and rely on Canon and their experience to ensure the gear that im using is top notch because they are the engineers, developers and masters of building a Camera.
If Canon wasnt into Photography they wouldnt be able to create the Cameras that they do today. I also respect that Canon isnt able to identify all possible issues but once an external pro photographer has located this issue and knows its present in the 5d3 and possibly other cameras, Canon should find a solution to fix this issue on all affected cameras because they as a Camera manufacturer with magnitude of experience wasnt able to identify this issue in the first place.
my 2cents, GLHF