Cost aside, it just complicates your workplace and makes your success dependent on the continued performance of other companies. What if slack pushes an update that everyone in your office hates, or shuts down as a company one day? In the words of Bane, it would be extremely painful, for you.

However, if you were old school and used IRC, you would have 40 years of documentation to help you solve any issues, and you probably won't have very much issues to begin with since people have been publicly using the protocol for 40 years and improving the experience. Plus it would be free and you wouldn't be dependent on anyone but yourselves staying in business.