Hi Miriam:
A good outcome for students is they can honetly assess without feeling "Oh, my grade can be affected by this test". If the activity is presented as non-graded, we'll get better responses to help us make our decisions.
For me, the key word is emphasis: What do I want to emphasize (based on the information obtained from the pretest)? Like you stated, it can save time by focusing on a topic students already are fairly knowledgeable.
Regards, Barry
