Well; I found that most drivers I had been looking at the scsi command payload isn't used at all; the drivers primarily cared about the (driver-provided) payload, and were completely ignoring the scsi command payload.
As mentioned in the cover letter response, it just seems best to keep the normal scsi_cmnd payload but have other means to add on the internal command data, like using host_scribble or scsi_cmnd priv data.
Similar for ATA/libsas: you basically never issue real scsi commands, but either 'raw' ATA requests or SCSI TMFs. None of which are scsi commands, so providing them is a bit of a waste.
(And causes irritations, too, as a scsi command requires associated pointers like ->device etc to be set up. Which makes it tricky to use for the initial device setup.)