The stability of the metallic phase for the electronically three-dimensional AC60 compounds is considered theoretically. A calculation of a possible metal-insulator transition temperature as a function of the degree of imperfect nesting of the Fermi surface is performed. We find that RbC60 and CsC60 are unique systems so far where the instability of the metallic phase originates from the nearly half filled conduction band and not from a low dimensionality of the electronic properties.