I think that Slack (or any other IM client for work, really) is just a tool, not a solution. Meaning that it requires some organization and thought, rather than installing and hoping for the best. At least to some extent (i.e. have no idea about companies with hundreds of engineers online), it works. Equally important it is to acknowledge what is suitable for doing on IM and what is not.

If you want to discuss some large changes, sure, IM is a bad place. Conversation will probably get interrupted, derailed, etc. That's where, in my opinion, things like Google Docs or plain simple GH Issues shine. The format forces into better thought out, longer messages.

For other things, you need to organize the chat rooms. If you have 10+ people and 2 chat rooms (I think slack defaults to #general and #random) then yes, sounds bad. But instead having quite a few chat rooms with <10 people each might work out well. Even having multiple rooms with the same people can be good to separate different topics of conversation.

"Always on", I believe, comes not from the IM itself but from the culture of the company. I know places where everything is e-mail only and people constantly refresh it. Equally, IM is mostly ghost town outside working hours at other places. And Slack has decent notification settings to make sure that you don't go crazy.

So, IM has it's place and provides value if you are willing to make it work. It's just not a 'it just works' solution, which is hardly surprising.