There are many cases where L2500 or L1900 signal is too weak to provide a reliable uplink, even though downlink still works well. In these cases, instead of placing users on L800, wouldn't it be convenient if L1900 or L2500 could be primarily used for downlink while L800 was used for uplink? This would save L800 capacity for real edge-of-cell scenarios where L800 is the only available LTE network.
I don't think L800 should be aggregated with other carriers in areas with strong signal strength, though. This would be unnecessarily wasting the little capacity it can provide. A setup where L800 could be the PCC but not an SCC would be ideal.