> They don't need it for performance because the SCSI/SAS protocol is much more efficient than SATA and so in most cases a write cache isn't necessary.
I think you are correct on every other point but I dont think this is right. SATA is pretty much the SCSI protocol as is SAS, they are only slightly incompatible for marketing rather than technical reasons. The big performance difference historically between consumer (IDE) and enterprise (SCSI) drives was tagged command queuing which is now very common in SATA drives as well although it wasn't so common in early SATA implementations. A tagged command queue allows the drive to implement an elevator which is a big win against a naive implementation without one.