Let’s say that like an increasing number of the disposed you are giving up the job search and hanging out your own shingle then you should consider the benefits of an IM.
In the start up phase getting good ideas out, and built on, is how things come to life. The ability to communicate with partners (or just a trusted sanding board) can be a great benefit.
It can also help you with sharing files, like say potential logo designs from you graphic designer.
A solid, full featured, instant messenger can even help you to get funding. Imagine being able to update your projections in real time for your potential investors.
Sure, you would be able to do it with emails too, but not as fast. You could even use the IM once you are up and running. Think about the benefits to your customer service.