I just finished my first IM. Learning these "Things" has been very interesting. I'm finding it takes a lot longer for me to get set up and get comfortable with each new tool than I think it should. As for IM in a library setting, it could be very helpful to the patron and could save time for both the patron and librarian. I could see where it could be distracting and even frustrating for a librarian with multiple responsibilities to add this to the "to do list".
From a personal point of view I would rather talk to someone rather than text or type ( ok, I'm over 50 and I learned to type, not keyboard). Besides I'm a terrible typist!
As a patron, I might use IM if I had a question I just couldn't figure out on my own.