The advantage over IRC is centralised, searchable history, along with the fact that people don't need to be logged in all the time to be able to ping them. Sure, you could set up a bot with IRC, but then you have a separate interface for it and nobody uses it so all your discussions practically disappear forever, aside from their results in other systems.
EDIT: By the way - $5/mo/user is a tad less than what an on-site Exchange user costs, and is the same price as Office 365 Business Essentials (which basically gives you an email account and Lync). It's not overpriced, IMHO, given how useful it is.
Does this bouncer have a log search interface, or does finding something that someone said a while ago amount to opening up a terminal and grepping, and hoping that nobody loses their logs? What happens when someone new joins the company and wants to see whether there's been any discussion on <whatever>?
It's not exactly a slick system, though it probably works so long as everybody in the company is a Linux software developer.
EDIT: By the way - $5/mo/user is a tad less than what an on-site Exchange user costs, and is the same price as Office 365 Business Essentials (which basically gives you an email account and Lync). It's not overpriced, IMHO, given how useful it is.