I would say it depends on the objectives. If they're anything like mine were, then it probably isn't feasible (ie, care for a patient with a trach, etc- there may not be a trach on the floor during clinicals!).
I would definitely focus on the skills they're currently learning, or finding an interesting case that they can observe. Also, I totally agree with students being abused as CNA's while at clinicals. These skills are usually required prior to, or as a condition of, starting nursing school, and nursing students aren't there to learn how to give a bath.
is to let them shadow a nurse (a nurse who is willing, and doesn't mind having a student), so they get a feel of how it's done, before letting them off on their own. Then, I would give them individual objectives based on their knowledge base. Explaining to them the basis and how to formulate a careplan would also help.
The best CP I ever had was the one who made sure we really "got it" at the end of the day. She went around the room and we each shared our patient's medical diagnosis, and then we etched out a careplan together, using simple, common-sense interventions (not the paragraphs in the careplan books).