Small environments can get away with using Global Updating, but most environments want to control exactly what data is replicated, when where why etc.
It works great in a small environment that uses SuperAgents in each segment; any more complexity than that and it begins to become more to manage with less flexibility.
Global updating automates replication to your distributed repositories and keeping your managed systems up-to-date.
If you rely strictly on server tasks for your repository replication and client tasks for your client updates you can control carefully exactly when these events occur (perhaps after hours when your WAN has very little traffic). A global update is event driven so if something gets checked into your repository (a DAT, a patch for VSE, etc) that triggers a global update you can suddenly have repository replication and client update tasks running when you did not intend them to.
As joeleisenlipz said it good to use in small environment....
and its depends upon the environment and how it want to be....