Piercings have never been allowed in most MA facilities for the reasons already given; however, with some of the locations that are becoming more common (navel, nipple, etc.) the piercing can't be removed because it can't be put back in (navel is one, in particular - I've been told the hole is curved, and basically impossible to put back in) or because it is still healing. In such cases, the best thing to do would be to tape them down, either with bandaids or medical tape - and if the person refuses, or continues to complain about being hit there, that tells you something about their level of commitment to training, as opposed to the piercings. At that point, the instructor needs to be clear with the student - on the level of "so if someone attacks you on the street, you're going to say 'stop, you can't hit me there because of my belly ring', or what?" Piercings are going to continue to happen - but they cannot be allowed to be a liability for students or instructor - in class or self-defense.
