I'm not 100% sure, so this is actually more of a question to someone who knows for sure.... but if you use exclusively SAS-3 HDDs on a SAS3 expander backplane, doesn't the link from backplane to HBA then able to provide 12Gbps (per SAS lane, so 4x SAS lanes means 48Gbps) of bandwidth between the backplane and HBA? Meaning, even if the individual HDDs can't saturate the 12Gbps SAS link, can the aggregate bandwidth of all HDDs in the backplane make use of the 48Gbps (6GB/sec) bandwidth from the backplane to HBA?
if the above is true, then another advantage of using SAS3 HDDs that can't saturate 12Gbps, is that it enables SAS3 backplane to operate at 12Gbps and provide broader aggregate bandwidth to the HBA while maintaining the advantage of an expander setup (simpler cabling, require less SAS ports/HBA cards, less power to power HBAs, etc.)