I once spent 6 hours on a conference call with two remote team members working to solve a time-critical software issue for a delivery. Using IM would have been cumber/tiresome and would have probably been much less efficient than voice.
Your right, that would have been cumbersome.
We use IM in my company for quick communication on projects we're working on or to give someone information without having to walk over to their desk and make them stop what they're doing.
Kind of like "where was the code for that thing you did last year", "hey, I'm out of data to review, can you send over some more", or "ok, I finished generating that report, can you take a look".
Doesn't work well for every situation, but most of my work is people working on the same task but very different parts. IM just serves as a way to communicate status in a way that would be really annoying in email.