I suspect that a lot of people are leveraging the simplicity of the Manage -> Create Attachments function in ArcCatalog, especially for capturing pictures with Collector. The rule for using any related tables offline requires that the Primary Key for the relationship tables be the Global ID field. (Prepare Data for Offline Use) When using Create Attachments, if a Global ID field is present on the table already, the relationship class will default to using the Global IDs. If not, it uses OBJECTID, a no-no for offline use.
My proposal is a warning prompt that mentions this rule before creating the relationship. Something akin to:
This table does not have a Global ID field, a requirement for offline data collection. Please add the Global ID field before continuing.
