In this paper we investigate in a-Si:H-based devices the accuracy of approximating dangling bonds by pairs of donor-like and acceptor-like states. We discuss the impact of using this approximation in device modeling by studying the dark current-voltage, the illuminated current-voltage and the spectral response curves. We find that the relative error introduced by this approximation in these characteristic curves can be tolerated when the correlation energy is assumed to be positive and when the capture cross-section of neutral states adopted is much smaller than that of charged states. A wide range of intrinsic layer-thickness values, density of states and temperatures has been investigated. This approximation fails when the correlation energy adopted is negative, and is not accurate enough when the correlation energy is assumed to be positive but the capture cross-section of neutral states adopted is higher than that of charged states.