Smart play. If they can lock people in to their free chat client they can get a stronger foothold into enterprise and servicing smaller start-up/SMB companies. This is what Paul Graham talks about building an e-mail client, just call it a todo list.

This is a storage application front-end. Slack and hipchat charge the money for secure storage, file transfer and data. That is Dropbox's competitive advantage and a great way to break into the market discreetly.

Client looks cool, I will be downloading it.