One of the few remaining astronomical bands (factor of ~ 10 in energy range) still without an all-sky imaging survey is the hard x-ray band (10-600 keV). This is in spite of sensitive imaging all- sky surveys already conducted at soft x-ray (0.2-2 keV; ROSAT) and soft/hard gamma -ray ( ~ 750 keV - 10 GeV; COMPTEL/EGRET) energies and imminent for medium x-ray energies (2-10 keV; ABRIXAS). A hard x-ray imaging survey conducted with wide-field coded aperture telescopes allows both hight sensitivity (and spatial/spectral resolution) and broad temporal coverage. We derive a generalized survey sensitivity/temporal parameter, Q, and compare previous and planned hard x-ray surveys with the proposed EXIST mission. Key scientific objectives that could be addressed with the enhanced capability of EXIST are outlined.