Actually, it's not a terrible idea to add an IDENTITY column to this table especially if it suffers a lot of inserts. If you make the clustered index (the default for PKs) on the column pairs given, you will constantly be fighting massive page splits that could actually cause timeouts for a GUI. I agree that the PK should not be on the IDENTITY column, but the clustered index should not be on the column pair... it should be on an IDENTITY column.
[EDIT] Please see my next post below for an exception to what I've stated above.
--Jeff Moden
Change is inevitable... Change for the better is not.