XChat appears to be lacking in one key feature: Support for ANSI color codes. I understand if you think they are never used. However, it is being used. And apparently, even mIRC supports this, and we all know how terrible that client is.
http://www.netswim.net/cantreadit.png for an example of what I mean.
I don't care what you feel the "need" for this is, if even mIRC, a terrible client, supports this standard, I fail to see why I must switch to irssi and lose an integral need (Python support) so I don't get a headache trying to read through garbage text.