Summary form only given. Radiative coupling between one or more quantum wells (QWs) and the single-cavity mode of a moderate reflectivity semiconductor microcavity leads to normal-mode coupling (NMC), observed as a doubled-peaked spectrum in reflection, transmission, or photoluminescence. Recent studies show that the excitonic response of microcavities depends very sensitively on structural disorder, e.g., influencing the NMC peak linewidths. A full treatment of the interplay of QW structural disorder, the attractive Coulomb interaction of electron-hole pairs, and the radiative coupling effects upon the optical response of real structures is a very complex task. We demonstrate here that the problem of disorder in a QW system can be isolated by carefully measuring the disorder-averaged optical susceptibility of a InGaAs multiple quantum well.