The second case, tenure helping good teachers stay in field—you're right on this in your review of Wendy Kopp's book. If the goal of getting excellent teachers to stick around for 20 or 30 years, then they need tenure protection in no small part because they are NEVER going to get paid what they are worth financially. Without tenure, teaching school cannot compete in the economic marketplace (e.g., I know people in the business world who have only a B.A. in business and, after 10 years in the field, are making 2.5-3 times what public school teachers are making. Without good job protection we will never have long term, high quality teachers in our classrooms. [emphasis mine]Everybody on the reformy side loves to say that "good" teachers should be paid more. Chris Christie says it. Arne Duncan says it. Bill Gates says it. Michelle Rhee says it. What they don't say is that that the "reforms" that they want to put in place are based around removing workplace protections that have an economic value for teachers.
In other words: if you're going to remove tenure and not replace it with some other sort of compensation, you are essentially paying a teacher less. It is worth something to that teacher to have tenure protections in place; it's foolish to claim otherwise. And if a teacher is getting less for the same work, Economics 101 suggests the supply of workers willing to do that job is going to shrink (unless, of course, we are willing to settle for less-qualified or less-talented teachers).
This is exactly why the wingnuts are pushing the meme that teachers are overpaid in the first place: even they implicitly acknowledge that these "reforms" are erosions of teacher compensation. So they try to develop psuedo-academic arguments that teachers are overcompensated in the first place. One of the many problems with their arguments, however, is that they themselves never attempt to calculate the worth of things like tenure.
This is all happening at the same time teacher pensions are under attack, and health benefits continue to shrink. Can you understand why teachers are demoralized? It's bad enough that all the joy is being drained from their jobs; their compensation outside of wages is simultaneously being eroded (although their wages are taking a beating as well).
There is simply no way this can continue without teacher quality paying a price. I know it's fashionable to say that "teachers should consider themselves lucky to have a job!" but you can only erode a college-educated professionals compensation so much before the theory of supply and demand kicks in.
So the question to the reformyists is this: is getting rid of tenure so damn important that it's worth coming up with compensation to replace it? Or are we just going to gut tenure and let teacher quality slide?
There is a price to be paid for screwing over teachers, folks. Is it worth it?