One, sometimes feverishly defended, rule of our technical irc channels is about not pasting code into the conversation. To show your code you are supposed to use some web based paste application and then copy the link into the channel. The need to use two separate tools, irc client and web browser, for one task, code review, is just a small annoyance - but eliminating those small annoyances is the only way towards really usable software. Many people have internalized the procedure so much, that they don't think about it any more and I am sure there will be some of them arguing that this is not annoying at all. But there are additional steps to do: choose the tool, paste the link, switch the windows and all this is a burden to the brain, even if it is processed in the background - it still consumes some brain resources.
And now imagine that you have it in one tool - not only those additional steps go away - but it also opens the way for new features like seeing a pointer to the part of the code that the other person is talking about, seeing his edits in real-time and commenting on them immediately etc.
This is why I believe in convergence of communication tools.