Write-stream IDs

People keep bringing up this question. To me, the answer seems obvious: because without the FTL, software bugs or misconfiguration can damage the hardware (when not doing proper wear levelling). With rare exceptions, hardware is usually designed to be safe from software bugs.

Sure, the firmware-level FTL can also have bugs, but when the firmware is buggy then it can only be the HW vendor's fault. With a fully software FTL, who takes the blame? Who foots the bill for replacing damaged hardware? This is also not a problem with software FTL on tightly integrated embedded devices, such as cell phones, because the hardware and software comes from the same place.