Portfolios give teachers an indication as to what aspects of the educational process are working for each, specific, student and which ones are not. As class sizes become larger, yet the population of special needs students increases, portfolio assessment allows teachers to give individualized attention to all students within their classrooms. But the use of portfolios can be time-consuming, because of their individualized nature, and the fact that the grading criteria and requirements must be slightly different for every single student. Portfolios do have their detractors. Some believe that they can be overly subjective, and allow students who are less strong to avoid learning critical skills. Portfolio evaluation is most suitable for "evaluating programs that have flexible or individualized goals or outcomes. For example, within a program with the general purpose of enhancing children's social skills, some individual children may...

Portfolio assessment cannot be used entirely as a replacement for standardized testing, given that students will have to face such tests as part of the state's evaluation of school districts in the era of No Child Left Behind (NCLB). They must also have strict standards for each individual student to demonstrate competency in key content areas, even if the standards for evaluation vary on a student-by-student basis. While they cannot be the only means of student assessment, they can function as a valuable part of the educational process, if appropriately used.
References

Sewell, Mary Marczak, & Melanie Horn. Use of portfolio assessment in evaluation. Alternative Methods. Retrieved March 9, 2011 from the University of Arizona website http://ag.arizona.edu/sfcs/cyfernet/cyfar/Portfo~3.htm