HipChat has a nice API that allows us to shoot notifications from TribeHR, our Linux production boxes, code commits, Salesforce, etc. to our company-wide channels. It's a nice communication tool to use to keep the whole team on the same page.
The integrations that Atlassian tools provide are optimized for large businesses. One bill, SSO, bulk administration of users, etc. From a usability perspective, I'm not a fan of the Atlassian platform over other stacks, but they cater to the people who tend to call the shots.
HipChat integrates with Jira/Confluence/BitBucket/Bamboo et al. It is the only reason we are using it vs a host of tools sticky taped together with different interfaces.
As your team growths you will see the value in having a single unified set of tools that cover a majority of your needs vs a host of different tools chained together each providing a different experience, education and logins.
For a small team, there's no reason (for us at least) to move away from Trello, GitHub issues, etc.