Main course or Dessert?

At the final student engagement session with Roland Case, from the Critical Thinking Consortium, we had a chance to look at some projects that were designed to support project based learning (PBL).
I wasn't very impressed with many of the projects, they were mostly research-then-summarize type projects, but some did offer some good ideas that I could use in my teaching.
The Buck Institute for Education is the leading source for information about PBL and they maintain that the best projects are those that are "The main course, not dessert". So without the project, the learning doesn't take place.
Traditionally many projects are done at the end of the learning, a chance for students to demonstrate knowledge, skills and attitudes. This seems to be more of what the Alberta Assessment Consortium refers to as performance assessment:

a meaningful, real-life task that enables students to demonstrate what they know and can do in situations like those they will encounter outside the classroom as well as in situations that simulate how people do their work

After viewing what some teachers are refering to as PBL projects, I think that I much prefer the performance assessment task. I teach junior high and it is difficult for my students to be given a task for which they are expected to do the learning on their own, independetly or in groups.
It would require a lot of time to ensure they developed the prerequisite skills to complete these projects. Students need to be very good at finding accurate information and then they need to be able to analyze and synthesize that information. These are not skills that many of my grade 8 or 9 students have mastered. I have tried these projects and they have not been very successful for the students who have difficulties reading, who are not self-starters and who have difficulties staying on task for sustained amounts of time.
I am in the process of transforming assessment in my classes (not by leaps and bounds, but it is in the process) and I have stopped grading students while they are in the process of learning. I only grade a task at the end of their learning, once they have had many opportunities to master the content and skills. With PBL it seems like you would be grading a product that was created during the learning process, which contradicts what I have been attempting to accomplish.
Or maybe I am just not understanding PBL and how I could use it to enhance learning in my class. So, for now, I will stick to the performance assessment tasks that allow students to demonstrate their knowledge and skills after learning has occured.
Everyone likes dessert better anyway, right?