It's only expensive compared to HipChat which is a subsidised gateway service that leads to Jira. Our company used HipChat for about a year before moving to Slack. Honestly, the differences are not many and any reasonable team could choose either.

I think we migrated to Slack for connectivity reasons, fonts and general user interface, and the bigger list of integrations. Are these worth $6 per user per month? If you've got the money then maybe. But it's by no means a given.