Especially for obscure questions, a larger list provides a better chance of getting a useful answer. Although the turnover in subscribers can be substantial, electronic conferences help people to discover new colleagues. They lead to the creation of simple communities that are connected by a common interest. Secondly, we widely recognize the important of academic freedom as the only way to ensure vigorous, creative approaches to understanding the world around us. I think that the right to academic freedom carries with it a professional responsibility to expose ourselves to competing viewpoints, even if they are antagonistic.
Every archaeologist works for the public in one respect or another.
You may also get useful ideas that are relevant to your own research from messages that seem to be on completely different topics. Permitting the free expression of controversial ideas does not mean that anything is permitted. Unmoderated lists have every right to establish rules against expression that is obscene and threatening. They also have the right to establish guidelines for participation so that the discussions are not dominated by a few individuals. With these rules in place, the potential for collaborative interaction and opportunity to learn more about one's field is enormous.