A better question would be to reverse the situation.

"What can I do on a kn7000 that CAN'T be done on a Motif?"

Many workstations have extensive editing and sampling, but no arranger features. Some feature elaborate arpeggiators that can give you a "psuedo-arranger" in a very rigid, techno kind of way, but to lay down trax...you need to do each part separatly.
eg:
drums(a pattern, or from scratch)
bass (Played live)
strings, horns, piano, etc......

get the picture?
Also, the pro workstations are usually bulit for road use and have better feeling key beds, and heavier cases. No speakers either.

A kn7000 has a terrific sound engine, but I've never gotten "inside" one to comment on the actual synth section. These comments are generalized to include my experience with hundreds of arrangers over the years. Hope something I said helps !

An arranger is a great way to lay down basic trax in a studio (or bedroom), and you can build on your progect from ther. Many players have trouble writing drum parts, and bass lines.....arrangers take all the worry out of that kind of task.

I have one arranger and at least one hefty module in my studio at all times. That gives me both worlds to use.