I'm talking about using slack as a member of various groups, with very different focuses. The fact that it is centralized means I don't need to have Yet Another Server I have to remember the address to, the username, the password, etc, and enter in on my desktop, my laptop, my mobile, etc. Plus, if one of those decentralized instances has a mobile app, chances are, I'd have to put in configuration info there for each and every service. Self-hosted communications services quickly become a pain in the ass when you end up needing to deal with more than a couple, and sometimes centralizing things a bit is an actual benefit.

You still have to create accounts on all of them. And if you mistype your password on one, you'll have to change it, and either you have to remember that there's one with a changed password, or go and change your password on all of them. And what if someone's taken your regular name on one of them?