Why bother with a Tyros 2 when a Motif EX with larger screen, voice effects, and chord recognition could do the job far better than any standalone arranger. This is a no-brainer. Why have two departments within a company creating two costly products, when one would do the job.
No one here could argue that an 'arranger' keyboard sounds like an arranger keyboard. Prove me wrong. Do a workstation song to a comparative arranger song and you will hear the difference.
As of now, technically there is NO need for arranger keyboards. It appears they are being made to keep the status quo and keep putting out old technology. Arrangers have not improved all that much in 10+ years when you think about it.