i dont think that you are making a fair comparrison. You are comparing apples with oranges. If you take the best musicians in the world and put them together i guess they would come up with some amazing music and would be playing freely and intuitively but their music would still have to follow certain rules. They would have toplay in the same key, in the same time, they would not all be able to solo at the same time, someone would have to do the "meat and potatoes" part of an arrangement so that the whole piece sounds tasteful.

The arranger keyboard does the meat and potatoes part and will never sound like Dream theatre . But as a player you can add spice and taset so that the entire performance demonstrates skill and training.

Yes there are greater restrictions and no it wont sound exackly like live musicians every time you play because you will have heard that 16 bar pattern many times over and will know that bass line pattern, will know that fill riff 1,2,3, etc and the only truly original element to what you play is you. but what you do within those constraints that i grant you exist is simply unlimited. Thats the exciting thing about arrangers.

I have played with live musicians who know absolutely nothing about taste and finess, who go at every note like their lives depended upon it. Drummers who think that hitting splash symols hard and often makes them great drummers even though they have no concept of time! But as some one who does regard themselves as a musician first(aspiring at least) who happens to play an arranger , i have ultimate flexibility when i use an arranger. I can play live with other live musicians and just the rhythm section or with rhythm and bass coming from my keyboard. Unfortunately many arranger players simply dont aspire to do much more than accompany the accompanyment. But dont blame the instrument. I have come accross mediocre bass players,guitarists too but i dont blame the instrument for the poor sound.

The ultimate arranger player is Jordan rudess but he would not describe himself as one. But he uses the technology within th oasys (and the kurzweil before that) just like an arranger player would use an aranger with registrations and samples that are set up and triggered when he plays certain keys. He may have created or sampled all the sounds himself but he triggers certain patterns just like we do.

Its just far more impressive and clever and less predictable when he does it