It seems like a no-brainer. If you want to evaluate my effectiveness as a teacher, you need to look at what I do in my classroom. If you want to evaluate my impact on student learning, you need to look at the work I make my students do and see how that work reflects my students' growth over time.
This is the right way to judge my job performance. But doing it this way takes time, is complicated for my boss (who has to do the same for two-dozen other teachers in two-dozen other contexts), and requires physical and intellectual investment into practices that can sometimes be uncomfortable and challenging (read: it requires change).
Too often in education, doing the difficult, right thing is avoided in favor of doing the simpler, easier to administer thing. When our students do this, we chastise them for cutting corners and missing out on the real value of the work–by doing so they are only cheating themselves, we tell them. When we do this as a system, we are cheating society.