If you feel that it could improve, contribute to the IRCv3 working group! http://ircv3.net/
Chat history makes chat more usable, and it makes it usable at all on intermittent connections.
As far as i can tell it would add no benefit but only overhead to the protocol.
It's like asking why would http protocol allow users to send data. Receiving is enough.
I like IRC because its simple. I've build a IRC out of boredom, and a bouncer because the one i used missed a feature i wanted. Please nobody take that simplicity away :/
e.g. https://bugzilla.mozilla.org/show_bug.cgi?id=687798
There's also an IRCv3 section on blog.irccoud.com but I can't access it from here.