The accuracy of the linear Cahn-Hilliard-Cook (CHC) theory in matrix form (multicomponent description) is scrutinized by comparing its exact macroscopic derivation with its phenomenological derivation based on the linear Markov process assumption. The effect of the coupling of density modes to those which are not directly observed is examined. The influence of the initial state on the accuracy of the CHC theory is discussed. Interpretation of scattering experiments using a bimodal description of transients is presented in detail. As an application, the possible effect of coupling to viscoelastic modes on transients is analysed.