Excellent points Nigel! I still feel it's a marketing thing with the lower end models to move those buyers (who were buying the lower end units for dual purposes) to the mid and higher end models.

It's also not just keyboards that have midi jacks, but tons of other gear use standard midi jacks as well. Here's another thing that doesn't make sense. The Yamaha MM6 (which is also an arranger keyboard) sells for $100 less than the S-500 and the MM6 has midi jacks (and) a USB midi interface.