This stuff seems so cartoonishly bad that I feel like I'm missing something. But I've been unable to find the progressive arguments. We need popular progressive bloggers to explain the other side.
So the big question is, why make everybody take the slow track? Why not offer the option of 8th grade algebra to students who are ready for it? As far as I can tell from listening to advocates, it actually is the Harrison Bergeron thing, but with a twist: the achievement gaps that they're trying to narrow have strong correlations with race and class, which makes closing those gaps almost a sacred cause. And that typically is where the arguments stop, in much the way that a mathematical proof stops when you say "QED".