It's like anything; ideally you'd use the right tool for the job.

IM, telephone and video conferencing are good tools for talking things through and getting quick answers.

One drawback with IM et al. is that it requires your immediate attention; it doesn't allow you to manage how and when you communicate as easily as email does. It also makes for a much more difficult audit trail; when miscommunication happens it can be much more difficult to figure out how things went wrong or what decisions were actually made and approved (an important thing for a consultant!) .

Email is better at communicating complex ideas and those things that require planning, thought and research.

Email also allows for a much more asynchronous form of communication, if you are busy you can simply look at your messages later. Along with this, it is a less demanding form of communication; if I IM, phone, or just show up at your desk I am coming to you with the expectation that you will stop what you are doing and give me your attention now, if I email you I allow the communication to happen on your terms.

Different jobs will have different demands so the appropriate toolbox of technologies will differ from job to job.