It would be very difficult for an attacker to insert a fake message into the system, as detection would occur quickly. The main advantage of this method is everyone has access to e-mail infrastructure, and the software for X.509 certificates is built into almost all mailers, PGP is cheap to free and relatively easy to install and use. Some services also support setting access lists for nicknames, so if you IRC from one workstation regularly you could associate that IP with your nickname, thus preventing an attacker from using it. The SSL solution is more intrusive to the software but requires less work at the OS layer, IPSec is the opposite, IRC wouldn't know about it, but the OS would need to be configured. IRC also has the added advantage of making file transfers relatively easy, which can be a boon to users.
Message boards are also popular and they have several advantages over mailing lists.
The primary reason being that everything is central and all that the users typically need is a web browser, meaning they can be accessed from public terminals if need be. Most mail clients also support Usenet news, which has the advantage of being more public than e-mail, being stored on a central server, and is capable of handling high volumes of messages. Usenet news servers can be setup so that authentication is required, and you can SSL wrap communications with relative ease. Discussion groups can be configured to be moderated which is mostly useful for announcement type things as opposed to discussion. When you moderate a discussion the time between messages goes up, which can be a problem, but the quality tends to be higher.