In this paper, we introduce a heuristic quantum model (HQM) for the permittivity of an emitter shell reconciling measurement data and theory. The model takes into account the influence of the plasmonic cavity onto the optical properties of the emitters. It is based on a semiclassical description of a two-level system in a classical electric field, but uses the vacuum Rabi frequency arising from the interaction between the collective emitter dipole and the quantized plasmonic mode. This results in the so-called power broadening of the emitters that suppresses the spurious peak. We compare the model to experimental extinction spectra of core-shell nanorods with systematic size variation, obtaining an excellent agreement between measurement and simulation. Our proposed HQM indicates that simple classical models do not capture important aspects of plasmon-exciton systems. It suggests that even the vacuum field fluctuations can modify the emitters' optical properties by inducing power broadening both in the strong coupling regime and well before in the weak coupling regime. The paper is structured as follows: First, we will revisit the electromagnetic modeling of core-shell systems and discuss the appearance of a third peak for the classical Lorentz model within the analytic example of spherical particles. In this context, we will propose and discuss a modification of the classical model via our heuristic quantum mechanical model that takes the two-level nature of the shell into account. After that, we will apply both models to experimental data for core-shell nanorods of various sizes. We find that the HQM recovers measured data with high accuracy while the classical model fails. We will then close by discussing the nature and the implications of the HQM.