This model is a a "toy" because it doesn't actually model any real physics, but simply parameterizes the scaling laws you might expect for halos in a scale-free cosmology. (e.g., cvir(M_*)=constant with redshift.) It's interesting that the model seems to describe halos in realistic cosmologies fairly well. A more detailed investigation into the origin of halo concentration is done by our group in Wechsler et al. (2002).