We use both because both have reason to exist. Since I started working in a multinational company spread across different sites (and continents) I have appreciated both approaches: sometimes the chat is a quick way to ask something, to verify that something is working etc.. compared to mail (that is not always seen "in time") or using the phone (as a bonus points, IMHO for non-english speakers it is easier to chat than to talk...)

I think IM is a bit of a misnomer when applied to products like Kato. Persistence, sync across devices, and instant search make it a lot more like "multi-person, temporally-organized Evernote, with real-time feedback", which is closer to replacing email than IM.

For what it's worth, the Kato team so far hasn't sent a single internal email over the history of the company. (We're distributed across ~15 cities.)