I always have to roll my eyes when I see most defenders of the tenure status quo whipping themselves into a righteous frenzy about how necessary it is to protect their freedom. I've come to the conclusion that tenure is valuable to most academics not so much because it allows them to take controversial stands without being fired, but rather because it allows them to imagine that their stands are so controversial -- so radial and upsetting to the powers that be -- that they need such protection.
I think such a long-term-but-not-permanent tenure system might also be beneficial to the truly radical scholars. (The obvious example here would be Andrea Smith, but I don't know the details of her case enough to directly apply my thoughts to her.) Certainly they would benefit more from permanent tenure -- but for that very reason they may be less likely to get it. If the tenure is only a 15-year contract, the university may be more willing to take a chance on a more status-quo-challenging scholar. Permanent tenure is such a momentous thing that it might be triggering (conscious or unconscious) prejudices that lead to second-guessing and eventual denial of tenure.