Service choice is something you have with email and, for the most part, with your regular phone service today. This means that regardless of whom you choose as your email service provider (Gmail, Hotmail, Yahoo! Mail, your school or ISP, etc), you can email anyone who is using another service provider. The same applies to phone service. You can call someone even if they do not use the same phone company as you do. This allows you to choose your service provider based on other more important factors, such as features, quality of service, and price, while still being able to talk to anyone you want.
Unfortunately, the same is not true with many popular IM and VOIP networks today. If the people you want to talk to are all on different IM/VOIP services, you need to sign up for an account on each service and connect to each service to talk to them.
The Google Talk network supports open interoperability with hundreds of other communications service providers through a process known as federation. This means that a user on one service can communicate with users on another service without needing to sign up for, or sign in with, each service.