Other than the support lifecycle and improvements with query performance and administration, there is another reason to upgrade your databases to 90 compatibility mode.
Compatibility levels are only available for the last two versions. For more information about compatibility levels, see ALTER DATABASE Compatibility Level (Transact-SQL).
Note that even if you have performed a SQL 2005 upgrade of your databases, the compatibility mode may still be set at 80 (SQL 2000) mode.
Best to run the upgrade advisor and have a disaster recovery plan in place before switching compatibility modes.