I'm not sure about the PRO thing... but I consider myself as one. Never used arrangers in real life, but traditional synths and piano. I play with a few rock bands, but also do some classical and other performances.

Anyway, a few comments about arranger keyboards:

1) First one I ever played (don't remember the model anymore) knew only 4 (FOUR!) types of chords: minor, major and respective 7ths. This really pissed me off.

2) Many arrangers know nothing about chord inversions (especially bass inversions). This also pisses me off.

3) Last week a gave a try to VA series, and I found it is capable of recognising all the chords I was able to play! (Including major7ths, ninths, 11ths, 6ths, dims etc)
Also, there's an option to put "bass inversions" on. (That is, IE. Am7/G, where G is the bass note) And that is something I love!
That's why I'm going to buy a VA-7 as soon as I have the money.

And "how" I'm going to use it? As an arranger. I'm going to do gigs with just a vocalist and my VA. Maybe a guitar player.

Also, I'm using it to practise solos. Just use it's 2-track live recorder to play the basis, and the replay it (looping it) and doing the solo over it (using a piano or a guitar or the VA itself)

Also, it's damn good help when I'm composing. I can instantly hear my ideas with the whole orchestra. This really is not possible with a real band...

And finally, it's a good way to audition my compositions to others in the band. I can compose a song at home, and then just take my VA with me, and play the song to the others (with drums, guitars, bass and keyboards so they will get a good idea about my song). That's much better than playing just the keyboard part, and trying to explain "I need that type guitar backing and the drummer should play that way..."

Enough said?

- Sami

PS. Why your teacher hates arrangers? It's the fact that arrangers are capable of doing chords by only a few fingers. You play just note c, and you get C Major. You play notes C and Eb, you get C minor. You play notes c and Bb, you get C7 etc. This doesn't do good for your piano playing That's why I always use real chords with 3, 4or 5 fingers.