DMZ 1.7.1 (DataMapper OverZealous Edition)

[eluser]OverZealous[/eluser]
@leo_
It is not necessary, but it is good database design. Every table should have a unique, dedicated primary key. For proper database normalization, it's not usually recommended to use composite primary keys. (I think at one time not all databases supported composite PKs, so having a dedicated PK also helped with portability.)

As you noted, DMZ does not currently use the ID. I still recommend it, and cannot guarantee that future versions of DMZ won't use the ID. There's no good argument for leaving it out, IMO, because it's a tiny integer field.

Finally, a really good argument for it, is when having multiple relationships between the same objects (scroll to Setting up the Table Structure with Advanced Relationships). In this case, there is more than just two columns on the table, and the PK is much less obvious.