It is a legitimate improvement to see everything that happened when you were offline. At that point you're not supporting XMPP; you're supporting "XMPP with some pixie dust to save that and let it be viewed, which is in a totally different place than your live chats".

Slack is a not-great implementation of a good idea, including a use case that IRC and XMPP don't meet. Tulip is a better implementation of that good idea, plus yet another good idea.