This article is a discussion on the necessity of the assumption of diffuse field in statistical energy analysis and the validity of the coupling power proportionality which states that the vibrational power exchanged between coupled subsystems is proportional to the difference of their modal energies. It is proposed to re-formulate the coupling power proportionality in terms of local energy density instead of modal energy. We show that this generalized form remains valid even if the vibrational field is not diffuse. Three causes of lack of diffuseness have been studied: coherence of rays in symmetrical geometries, nonergodic geometries, and the effect of high damping. Numerical simulations and experimental results conducted on flat plates in flexural vibration are provided to support these statements.