The Exchange topology is simplified.
You can administer Exchange centrally and reduce administrative costs.
You can make better use of hardware because there are fewer mailbox servers as well as fewer auxiliary servers. A centralized datacenter can also increase scalability and availability.
Consolidating sites can help your organization reach the goal of running Exchange in native mode by reducing the number of Exchange 5.5 servers in the organization.
With fewer mailbox servers, there are fewer targets for security issues.

