...especially when it can be easily added (created) with a ADDPFCST command even if there is no key in the DDS.Be careful here... you can't add a *PRIKEY over a file if a non-unique key already exists. So in order to have the primary key constraint you must either remove the key from the file or make it unique. In any case, the uniquekey processing adds considerable overhead (running the same program over the file after removing the key and adding the constraint caused a runtime of 9100ms - nearly the same 7-to-1 increase I saw with UNIQUE, which is to be expected since they're probably implemented much the same way).
The only point is that it's such an easy process to add a primary key that it's essentially trivial. It's almost as easy to remove the constraint later.As long as you don't already have a key and you're willing to take the performance hit.