Turning on logging in most IRC clients is very easy. But once again this is an extra step and means learning the client a bit. One nice thing is that I can have those logs offline and search them whenever and however I like. I regularly do search my irc logs to find that links I didn't save, or re-read an important discussion.
Most things in slack are available in some form with irc, but not preconfigured out of the box. And some things on irc can take some fiddling to get set up right, and you can't count on everyone else to have enabled support.