I am currently a member on seven Slacks. Each one has a different username and password combination. I have to log in to each one separately. AFAICT there is no way to link them so I can log into all of them all at once. And then once I'm in, every Slack has multiple channels. Trying to keep up with all of them is hopeless. But if I'm not paying attention to a conversation in real time, going back after the fact and trying to extract meaningful content is difficult (it's hard just to find where a particular conversation starts and ends). Trying to go back and make a contribution to the conversation after the fact is completely hopeless.
Yes, real-time chat can be very handy, but I don't see anything that Slack does that Adium or Skype doesn't do better IMHO. And for archival discussions I'd much rather have a Google group. Keeping a threaded structure makes it so much easier to go back and trace the conversation after the fact.