Related tables need to be supported in Collector. We employ an asset hierarchy of processes and assets that is about 5 tiers deep. In ArcMap you can view each tier by expanding the plus sign in the identify window. ArcGIS Online only shows the current tier you've clicked on, and Collector not at all. As a result, Collector is not a viable solution currently for field workers collecting assets who would like to see all the tiers. They need all the tiers visible to create the appropriate relationships between all the tables.
Moderator's Note: This idea was originally submitted by Phillip Sherman for both ArcGIS Online and Collector. This new idea is modified to be specific to this functionality in Collector for ArcGIS.