If the IsAggregatable property is set to True, an (All) level will exist. A hierarchy has no (All) level if the IsAggregatable property is set to False.
The presence of an (All) level in an attribute hierarchy depends on the IsAggregatable property setting for the attribute and the presence of an (All) level in a user-defined hierarchy depends on the IsAggregatable property of the attribute at the top-most level of user-defined hierarchy.
Note - also that setting it to false means that the default member, if not defined, will be the first member in the list of members for that attribute. As such, any query will "Exists" any hierarchy selection from that dimension with a member from that attribute when the attribute is not queried explicitly.
Learning Office 2010 + SharePoint 2010 + SQL Server 2008 R2