Eli,
Personally, I’ve found Slack useful for real-time, shared planning among disparately-located people. For example, I can see it being useful for facilitating questions in real-time from new LSP facilitators, running Q&A or virtual information sharing sessions. It also allows for team management and file sharing. It has been most useful in assisting collaboration among geographically-dispersed teams. I’ve also seen/used it coupled with group video conferencing like hangouts, Skype, etc.
Personally, I don’t see it as a replacement for the forums, but another channel to facilitate communication in a more synchronous way.