http://www.inc.com/jeff-haden/the-company-that-outlawed-emai...
Key highlights from INC article:
>“While email makes for a decent communication tool with clients, internally it doesn’t facilitate collaboration and basic workflow. Email has no intelligent mechanisms for prioritization, lacks context, lacks a framework for knowledge management, and saps accountability.”
>“We’ve all been on an e-mail thread where people are answering questions but really just creating more confusion,” Segal says, “or maybe referencing some missing yet critical attachment. So we looked for tools to help us better manage our basic workflow.”
>Knowledge can get lost in a person’s mailbox and prioritization is basically nonexistent since inboxes assume that most recent equals most important.
Also, another point to think of: when one of those tools you use go away, how do you salvage your progress / information? It's not bad to try to minimize email usage, but I do find it bad to rely on external services for such critical stuff (meaning the documents with information).
Why not use a selfhosted option? That way you can be sure that it won't go away anytime soon.