I worked for a large news company that moved from virtually nothing to slack. There is now something like 800 channels.
Each channel has a purpose that is distinct from each other (for example system-outage-44, or graphics-team, or aws-help)
Each team normally has a public and private channel, one to provide support, the other brisk frank discourse. Seniors are expected to contribute(in fact the members of the board are on there), and its the only way that remote workers can get real time collaberation. In fact it allowed people to work at home and feel included.
So to say its a waste of time sounds like either a marketing ploy, or an organisational problem.