Combined interferometer and polarimeter systems, using a single detecting element per line of sight, are susceptible to perturbation of the interferometric phase when modulation of the polarization vector is applied. This issue has been investigated extensively for the case of a rotating elliptically polarized probing beam, demonstrating that here a perturbation is inevitable. In this article the analogy between this analysis and earlier work is pointed out, and the underlying physics discussed. It will be demonstrated that schemes have been proposed in which the perturbation has been avoided or kept well within acceptable limits.