You could use Candy (http://candy-chat.github.com/candy) as a nice web UI for XMPP MUC (Multi-User-Chat). It supports basic features but is easily extendable through plugins.
The one thing that has the lowest utility is a web based interface to chat since it essentially doesn't work well in the background in the same way as an app like Pidgin does.
Hipchat is great at what it does, but without audio and video support, I'd never deploy it in the office.
The big advantage I see to IRC is how long it's been around. Plenty of clients and setups catering to IRC. And it's not going away anytime soon. Perhaps it's getting a little long in the tooth, as posed by moe at http://news.ycombinator.com/item?id=2069810
I have too much free time.
Or maybe I misinterpret the license?
Some applications do struggle when running behind a proxy (I'm looking at you Jira) but my experience tells me that it's harder to break SSL support than not.