We propose a simplified description of fluid adsorption on heterogeneous micropatterned substrates. Using this approach, we are able to rederive results obtained earlier using effective interfacial Hamiltonian methods and predict a number of new examples of surface phase behavior for both singly and periodically striped substrates. In particular, we show that, for a singly striped system, the manner in which the locus of surface unbending phase transitions approaches the prewetting line of the infinite pure system, in the limit of large stripe widths, is nontrivial and sensitive to several characteristic length scales and competing free-energies. For periodic substrates, we investigate finite-size deviations from Cassie's Law for the wetting temperature of the heterogeneous system when the domain sizes are mesoscopic.