They are using the WS just for SMF backing, though, unless they are doing trance or hiphop loop-based musics (that WS do very well). There is no interactivity with the backing on a WS, you just play over it karaoke style...

THAT'S what arrangers give you... the flexibility to change anything and everything DURING a song, while you perform. WS's still trail that ability by a wide margin.

I think a simpler advice to give is... if you are doing modern dance musics (trance, house, techno, hiphop and rap) you need a workstation. If you are playing older musics than this, you need an arranger. That was easy, wasn't it?!