In the core vocabulary we define a type definition for terms used as tags as:

<TypeDefinition Name="Tag" UnderlyingType="Edm.Boolean">
<Annotation Term="Core.Description" String="This is the type to use for all tagging terms" />
</TypeDefinition>

The intention of this was to allow the mere presence of a term (such as IsLanguageDependent) to be true.

However, we neglected to define the default value as part of the TypeDefinition, so each use of the Tag must ALSO specify the default value of true (which kinda defeats the purpose).

Perhaps this was because we don't currently list DefaultValue (or Nullable) in the allowable facets for a type definition (why?)