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As a  general principle it is best not to go overboard on defining SharePoint content types. They add power to information retrieval but also add content creation overheads. Keep the number of types reasonable and also the number of metadata fields. (Obviously the art is defining what ‘reasonable’ means)

A list of reasons to define a specific content type:

• you want to attach a document template for that content type
• there’s a standard workflow for that content type
• there’s a standard info policy for that content type
• you want properties of the content type to be possible to search through advanced search
• you want to restrict a search to that content type
• you want to be able to sort a list or library by a specific metadata field of the content type
• you want to categorise a list or library by a specific metadata field of the content type

See also Microsoft’s Managing enterprise metadata with content types