The problem with using the phone (and yes, I know you're going to think this is ridiculous but bear with me) is that if I'm working on anything text/screen -related and I get a call, I have to "shift gears" rather significantly and focus on the call at the expense of the other work that I'm doing. In other words, I switch from text to voice. When I get chat requests (through MS Communicator here at this time), I can more easily respond, either right then or by letting the request sit a couple of minutes.
Take a call from someone and see what they do when you don't answer their question for a minute or two.
So chat in general is more flexible for both ends, plus even if your recipient isn't there, you can send a question/comment through the IM protocol and it will be there when the recipient returns (assuming there's not, say, a crash or reboot).
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