Skype also requires download, creating an account etc etc.
Not everyone can join a skype conference chat.

IRC is alot more secured than your making it seem. IRC Chatrooms can be accessed by anyone through an IRC client, or simply from the website. IP Bans can be worked around, it is a bit troublesome to do and usually not worth people's time to do, specially for such a small group.

The IRC network it would be connecting to has 4 expierenced administrators who monitor every aspect of the network overlapping eachother to provide full coverage. It is heavily secured to prevent DDOS attacks, flooders, spammers and other maliscious users.
Skype is highly unstable compared to IRC wich has been around for decades. You may be able to click and remove that one user, assuming you are online at that time of course.

I naturally vote IRC because it is easier to access, more user friendly and way more secure