We presented the computed dielectric, ferroelectric and piezoelectric properties of four classes of plastic ionic molecular crystals. Several of the materials display large shear piezoelectric responses. Our analysis shows that molecular rotation in response to shear stress is key to understanding the piezoelectric properties of plastic ionic crystals. On one hand, molecular rotations can allow for the unit cell to easily deform, i.e. provide large compliance coefficients. On the other hand, molecular rotation can both rotate molecular dipoles, but also induces related to intermolecular-charge transfer. This finding highlights the role of intermolecular bonding nature. The molecular rotation mechanism causing large shear responses reminiscent of the polarization vector rotation in inorganic perovskites, but while this mechanism is related to rotation and twisting of linked octahedra in the inorganics, the rotational freedom in the plastic inorganic crystals are linked to the dominantly non-covalent bonding nature. The fact that piezoelectric shear coefficients are comparable to phase pure inorganic piezoelectrics coupling with low dielectric functions make plastic ionic crystals promising for application in piezoelectric devices, such as energy harvesting and actuators. In particular, [Hdabco]ReO 4 and HQReO 4 have large energy harvesting figures of merits in their phase pure form, heralding great potential of this materials class with further material optimization.