The use of Ms Messenger on a network

The lead architec decides that msn messenger is the ideal tool to communicate with everybody in the departement. What are the pros and cons of using Messenger on a network.

Here are a few Pros that I’m given.

Easy to communicate with others,
Don’t have to get up as often.
If somebody works at home he can communicate qith the others at work.
and a few more.

What do you think? What are your experiences. Is it a major security risk?