We show that state-of-the-art tube models overestimate the influence of contour length fluctuations (CLF) on the apparent plateau modulus of entangled flexible polymers. The observed inconsistency between the molecular weight dependence of the relaxation time and plateau modulus questions some of the underlying physics and suggests, in particular, that the CLF-based explanation for the zero-shear viscosity vs molecular weight scaling law is not entirely satisfactory.