Very good point. Would also make spotting undercover cops easier.
Finally, we'd be at the point 90ies Warez kids thought we were at. "If he wears no badge number on his back, he can't be a cop, otherwise nothing can be used in court. We're safe, boys".
>>luckyl+(OP)
This is the least charitable interpretation of GP's proposal possible. A small amendment to GP's comment to apply only uniformed officers operating in public—as the D.C. troops from the article are—negates this reductio ad absurdum.