The major advantage of using a non-proprietary protocol like IRC is that people can choose their own clients... but the state of most IRC clients nowadays is abysmal. Colloquy on OS X, for example, hasn't been updated in years.
If I was going to use IRC to communicate with my team, I'd just create a private channel on Freenode. We'd miss out on archiving, but I don't think that that feature alone is worth $2/user/month.