From what I've been reading on here, Xchat has been difficult to compile, and anyone looking it up can find that it hasn't really been maintained, the last version coming out in 2010. I think it's rational then to have Hexchat included in mainline slackware instead of Xchat. Hexchat has shown intent to maintain the project, and while starting as a Windows only deal, now works on Linux as well, as a slackbuild of Hexchat indicates.
To me I consider this logical on the grounds that we moved to MariaDB on similar rationale; uncertainty of the future with the existing in favor of a fork with clear intent to maintain, except in this case Hexchat is that much more.
Here is an announcement on Hexchat's site with some more points: http://hexchat.org/news/announcement.html
What do you guys think? I think it's a rational thing to do.