Portfolio Based Assessments...Yea or Nay?

To be honest, this is the first time I have heard about portfolio based assessments. I've always been graded on what I did at one time, for a test or a paper. I've never had to collect everything I've ever written and turned it in to be graded.
So as Jessica and I sit in Panera she explains the assessment to me. This is what I've gathered:

• You may or may not have to turn in papers during the year. Instead the assignments are assigned and you work on them throughout the year with the teacher.
  
• Then you collect all your work and make a portfolio that you turn in to your teacher that contains the best editions of what you have written.
• The teacher then grades it as does a state grader, and they average the two grades.
  
• The teacher can challenge the grade of the state grader and bring in a third, but then the third grader has the final say on the grade of the student's work.

The Pros:

1. This gives the student chances to improve their writing (I'm speaking for Language arts) and fix mistakes that they might have made. This gives them a chance to fix their mistakes and produce a truly great piece of writing.
2. This also allows a student to challenge themselves to create a portfolio. It requires time and a lot of planning and organization skills. It helps the student to become more efficient with their time, etc.

I can see that their are Cons to portfolio based assessment, but that is for another time.