Just to add to that, "It Depends".
If this table is a lookup table that is mostly static, leaving out an IDENTITY column and using the the two columns as both the PK and the Clustered Index would be the way to go especially for performance.
Like I said previously, if this table suffers a large number of INSERTs, then add the IDENTITY column and make it the UNIQUE Clustered Index and make the PK Non-Clustered on the other two columns.
--Jeff Moden
Change is inevitable... Change for the better is not.