Avoid defining non-aggregatable attributes in a parent-child dimension with the exception of the parent attribute
This rule analyzes dimensions with parent-child hierarchies to determine whether all regular dimension attributes are aggregatable.
To increase usability, you should typically not define any regular dimension attributes as non-aggregatable. A dimension attribute is aggregatable if its IsAggregatable property value is set to True.
The default members for non-aggregatable attributes will not be at the (All) level. This will affect the result of queries in which the attributes are not included. Analysis Services uses the default member to evaluate expressions when an attribute is not included in a query. Because parent-child hierarchies generally represent the most interesting exploration path for users in dimensions that contain them, it is best to avoid having non-aggregatable attributes other than the parent attribute.
For more information about how to configure the IsAggregatable attribute property, see Configuring the (All) Level for Attribute Hierarchies in SQL Server Books Online.
For more information about performance issues with parent-child hierarchies, see the section, "Parent-child hierarchies," in the SQL Server 2005 Analysis Services Performance Guide.