Rollup releases are very useful in organizations with complex and nested product lines and products. A top level product may have a V2.0 which is a rollup of three subsystems, a perfect use case of a rollup release. If each of the subsystems, in turn, has sub-sub-systems with releases that need to be rolled up, they would benefit from having their own rollup releases. If the highest level rollup release could contain rollup releases below it, this would really add value.
An alternate implementation would be to allow any one product release to belong to more than one Rollup release. Today, a product workspace release can only belong to one rollup, which is not workable in the scenario where the product release is a member of the rollup release of its product line and also a member of a rollup release higher in the overall system hierarchy.