This model requires two things - a very competent Director of Studies who herds the TAs, and is also requires sufficiently competent undergrads. One would think that at Yale this will be less of a problem, but at lesser institutions, where half of the incoming class has problems with mathematics at the level that is required there will be problems. And that is why the Director of Studies needs to be given some authority to throw out incapable TAs, or else there will be a case of blind leading the blind.