Why in the world? You'll have no trouble finding videos of police blatantly misusing them in broad daylight. There are no consequences because... there are no consequences. Why would serializing rubber bullets help? Are we going to collect handfuls of them off the ground and then conjecture as to which officer fired which bullet?
They're not penetration rounds, so will likely ricochet off the individual. After which, they'll co-mingle with the rounds scattered around by all of the officers. Except in isolated cases that don't involve multiple officers, but then the serial number is a moot point.
So while a serial number can prove that a particular round came from a specific officer, proving a direct connection between any specific damage and a specific round would be an area of uncertainty and conjecture.
And even if you do keep track of an individual round, the above argument itself would allow for officers/lawyers to argue that case anyway, introducing enough reasonable doubt to weasel out of repercussions.
