I'll probably bring this up again in my coming Overview of Slack Alternatives (yes, I now owe you such a show!), but before I forget:

The problems with naked XMPP, with multiple clients and with clients coming and going, are these:

1. If nobody is online at the moment, your message will be lost in cyberspace, and you may or may not be told that this happened.
2. If you have several clients online, they have priorities set to determine which client should get the messages delivered to it.

So what these XEPs do is that they add the ability to:

1. Store-and-forward, so that the server holds any incoming messages, and delivers them when you get back online.

2. Carbon-copy, which means that all clients currently connected will get any incoming messages, rather than just one of the clients.

3. Message storage, which I'm not sure how it's handled, but I suppose the server can back-fill a connecting client so that any messages received since last time will be sent to it, even if other clients have already received those messages.

I'm just inferring this from comments and from the titles of the extensions, I haven't actually used them.

Now, talking about "naked XMPP" is probably not fair, because these extensions are supported by several XMPP servers, including the original ejabberd project and the rather popular Prosody project.