It was initially designed specifically for the Tyros2's new SA Voices, although the Combo Division, at that time, also wanted (and got) it for their Motif line.
That's the line got me curious, Ian... you said it, not me.
What about the SA voices NEEDED a better action?
Nothing wrong with being curious, Diki.
Let's just say both combo and arranger divisions used similar forms of articulation so sharing the same keybed was a natural.
As I said above, I have no idea why it remains exclusive on TOTL (for synths as well) only, other than to say it must have to with cost, but it does makes perfect sense for the top line instruments to have the best action available.
Look at it from your perspective....what prevented Roland from putting the keyboard action from the G-70 into the BK-9?
The action was already fully developed and proved, R&D was probably already covered, but yet, they didn't use the best they had in the BK-9, which is now their top arranger, which features many SuperNatural sounds that would definitely benefit from a higher quality action with aftertouch
I'm not a product developer, not even of the armchair variety, but I would hazard a guess and say the two situations would be related to cost and profit.