This should be done EVERY SINGLE TIME you are sending out a mass email for anything ever. People do not appreciate mass mailings that compromise their privacy.
I just joined relay for life for the American Cancer Society and the only option to email people is to put every person I want to email on the "TO" field. There isn't an option for a "BCC" An organization that large should be well aware of people's privacy and know full well that this is unacceptable.