I'm looking for specific information on whether the ESRI suite requires Oracle Spatial for any of the functionality, or if Locator is always sufficient for working with SDO_GEOMETRY objects? Do you know of any ESRI documentation that discusses the minimum requirements in Oracle for working with SDO_GEOMETRY objects in the ESRI suite?
Esri has no control over Oracle licensing policy, they just allow you to use the SDO_GEOMETRY datatype, as provided by Oracle. Some of the SDO accessor functions require more than just Locator. Your determination of what is sufficient should really be based on what SQL manipulation you require, since Esri doesn't use more than SDO_FILTER.