They think it will mean a loss of support, closing schools, and so on. Well, where the charter schools, or schools of choice within the public sector, seem to flourish best is in districts where there is a rapidly growing student population. If these schools of choice are different in the right direction, the kids will benefit, and nobody's losing their job. So of course that resonates back on the old schools. You mentioned the power of exhibitions.
Many people now see performance assessment as an important reform strategy.
The problem may not be the assessment strategy, but the insistence that it be done on a mass scale. And portfolio assessment is nothing more than taking the real work of a child seriously and looking at it over time, instead of just relying on some 30-minute writing sample. The problem is when you try to move from that to making comparisons between Johnny Jones in this school and Suzi Smith in that school. That's extremely difficult work, because reasonable people disagree over the quality of writing of an interesting paper. It's really tough to be consistent in grading when you get into rigorous academics.