We actually use slack (and hipchat, and jive, and whatsapp, and groupme, and skype, and who-knows-what-else.. ugh) for communications that we would consider sanitized for public consumption (so, no discussion about client IP, for example). The only approved chatroom/im tool that we use is GChat, though. The reason for that is that our internal team is comfortable with Google Apps as a (relatively) secure platform for communication.
They also approved of screenhero (I can guess why, but it would be a guess).
They have not approved of slack, hipchat, etc. for client-related communications because they are nervous about having that much liability not under our control. It also makes it difficult for our contracts team that would then be required to explain that client data could now be in either Google Apps or Slack.
I would say that these decisions are going to be SOP for a consultancy of a certain size.
I don't necessary agree with all of these decisions, but I understand and try to empathize with the other parties that would be inconvenienced.
Edit:
I think what happened is that screenhero got a verbal and passionate following within our consultancy that the decision to use screenhero was 'encouraged' into being adopted. We started using it under-the-radar when it was free and when it became paid, the organization realized how many people were using it successfully. It was a ground up effort.
It makes me sad that a tool we found so useful across our organization (which you can see above is incredibly fragmented) is being made inaccessible. I know you're trying to do right by your users and we're just a weird edge case, so no bad feelings. Just wish it were not so :(