Similarly a portofolio evaluation reduces the inherent unfairness that comes about because of the the lumpiness in publications that occurs in disciplines in which output comes about in large, intermittent chunks and that can distort scholarly output. Sure some people will quibble and fight for everything, but norms can and should develop. People don't go up for full capriciously, they test the waters, they have a sense of norms, and they understand what it takes to succeed. Plus being turned down year after year becomes embarrassing.
People plan their move from associate to full and more or less figure out when they are ready.