Is there any benefit of a primary key that comprises all columns of the table?
In your case these fields are natural key.


  Surrogate Key:
  
  Surrogate keys are keys that have no
  “business” meaning and are solely used
  to identify a record in the table. 
  Such keys are either database
  generated (example: Identity in SQL
  Server, Sequence in Oracle,
  Sequence/Identity in DB2 UDB etc.) or
  system generated values (like
  generated via a table in the schema).
  
  Natural Key:  
  
  Keys are natural if the attribute it
  represents is used for identification
  independently of the database schema. 
  What this basically means is that the
  keys are natural if people use them
  example: Invoice-Numbers, Tax-Ids, SSN
  etc.


Surrogate Keys vs Natural Keys for Primary Key

I prefer to add surrogate key to separate business and database model management.
Other question is using clustered and nonclustered index on primary key..
If you table changes (non static table, it has high intensive inserts or update), you will get problem with performance in case of using clustered index on non-monotonic increased key.