The other argument out there is: fighters are not going to want to wear shields (the Tie Domi and Chris Simons of the NHL). This can be sholved by taking the helment off before engaging in the fight with your opponent (ala Prpich and Peluso). There is also a NHL rule that if you start a fight and your wearing you will recieve an extra 2 minute penalty.
One other point, is some of the players in the NHL consider players that wear shields less tough than guys that don't wear shields. Just two lines of thought. Don Cherry has also eluded to this fact on the coaches corner. In fact I neither agree or disagree with these lines of thinking (I am not even sure where I stand on this issue), however, I do see the liability if one is not worn.