I think they're using comet for the requests so while the activity may be high when people are typing, during breaks it should be very low as opposed to ajax polling.

I do think that real-time is really needed for many applications. The few times I've tried collaborating on a google doc, I've given up because the lag introduced the possibility of overwriting each others edits. That and just a general annoyance with saying "Do you see it yet?" over and over.