It's a lot easier to write a functional IRC client by glancing at the RFC and banging on a piece of code until it mostly works. For example, http://lists.canonical.org/pipermail/kragen-hacks/2008-Febru... was written that way. Given that the only protocol message it has special handling for is PING, though, I'm pretty sure you could have written it based solely on the RFC.
However, I think it's a big mistake to claim IRC "scales amazingly well". The biggest IRC network today has tens of thousands of users (at the moment, freenode has 64000, undernet has 58000, and EFNet is down in the 32000 range) and the IRC networks are constantly suffering from breakdowns from overcapacity. Compare this to Skype, Facebook, or Gmail, with tens of millions of concurrent users.
I challenge you to write a functional irc client based solely on the rfc.
I've actually written IRC bots mostly from the top of my head.
The protocol and semantics are really simple.
Type this into a console near you:
nc irc.freenode.org 6667
USER foo bar batz boo
PRIVMSG #testchannel hello world
PRIVMSG test345 hello self
Yes, that's all it takes for a minimal, functional client.
(just remember to type PONG every once in a while)
I'm not sure what you mean by fractured. Like every protocol it has a few rough edges, but those are nowhere near the semantic nightmare that I witnessed when trying to dabble with XMPP (which admittedly was more than a year ago).
I remember attempting to write an IRC bot a while back and finding that RFC severely lacking when it came to connecting to whatever irc network I had chosen to test it against. The problem was that the handshake to join the server was different in the RFC than what the server was expecting. This sentiment was mirrored by others I consulted with over IRC that were devs on the epic3 irc client. So, while writing a IRC bot may be simple, I would imagine that writing a client isn't. The protocol itself was the same (in the "COMMAND args" sense), but the conventions differed.