In our program it varies between semesters. Our clinical objective in the final (6th) semester is to organize the care for 2-4 stable patients. As such, we certainly expect students to prioritize and use the nursing process to do it. But we only require one Care Plan.
We are struggling to answer some of the questions you raise. We don't have the answer. My feeling is that the Care Plans are not teaching the nursing process. I've taken to weekly abbreviated assignments that require students to take one compelling problem from one of their patients, then to outline all pertinent assessment data, state the problem (for this I do not require NANDA, neither do I allow them to use medical dx's), articulate their goal(s) (quantifiably measurable), outline their nursing actions, then explain how their plan worked (if it did) and (if it didn't) what they would recommend to be added, subtracted or changed.
I give the students feedback, then collect the mini-plans, and keep them on file. I can see if their use of the nursing process improves over the 16 weeks.
Maybe this will pan out, maybe not. But I'm with you. Formal Care Plans take massive amounts of work by the students and their clinical instructors, and I'm not seeing any payoff. If anyone else has ideas... I'm all ears!