My guess is that, since the 'original' models only had 2.4X writing, they want to keep that as a feature-- so when people check for new firmware, they see that is an option. If it ever went away, new buyers (or people who never thought to upgrade their firmware) may not know of that HUGE improvement. (being serious!
Anyone else think it's weird that suddenly the firmware is all 'unified'? Throws a bit of a monkey wrench in some of the theories as to the difference, although the files do seem larger (I wonder if it's 2 slightly different firmwares that get loaded depending on if you have an "X" model or not?).
Strange. I wish Sony would just own up to what's going on, but I'm sure that'll never happen. All I know is my DRU500A works great, as does my dad's.