Kolhell... there are several problems with getting a Java IRC client...
1. Java is HELL. Worst programming language on Earth, and I know myself and many others refuse to use any programs written in it. I woudl rather use the .NET runtime than the Java runtime.
2. The issue isn't bandwidth. Its database access. Flashchat takes almost no bandwidth, since all it does is relay text to and from the database. The problem is that it relays data to and from the database at a much higher rate; and each person who joins the flashchat essentially adds a whole fractal dimension of database reads.
3. IRC does not integrate at all with the user database. Instead of having a unified login system where people use the same login on the forums as they do in the chatroom, they would have a different login for each system. This becomes problematic since many times usernames would not be the same, and people could potentially impersonate other people. IRC is a security risk, and it has been since its beginning.