I just like jabber because it's a standard. I have an app on my phone that'll do Jabber already. There's a lot of server software already out there to host Jabber, it's pretty robust, all we need is a reasonable frontend for desktop
The counter to that—and no doubt a factor for many like Slack—is that when you use a custom protocol/API you get to control the whole experience. You don't have to deal with bugs in third-party clients, wait for clients to get emoji support, and can control the look-and-feel (many of the Jabber apps I've seen aren't great).
This is obviously not ideal for everyone, but I suspect that outside of tech that using a custom client is possibly even a plus.
