Hi:

Arrangers comparation to workstations, is like comparing SUV with Sports cars....they are very diferent and for diferent purpose.
Worksations are design for a musician that either work for a band/home or in a studio work, you can layer secuencing, looping and sampling, depending the kind of project. The similarities you mention:

Example Korg Triton EXm in the "grooves" or almost style like that worksations have, do not have an intro/s, variations, fill ins, and/or endings, that is why is very diferent. So, you do not have "control" over this, to recall a voice (strings as an example), in the worksations is much likely that you have to call a Bank and then a "patch" number....
In the Arrangers (Korg PA1X Pro), the voices (programs or patches) are group in families of the sound, Piano, E,piano, Strings, etc.....so you do NOT have to know a band and patch number.

One great diference for a songwriter/arranger, is that you can make a midi file of an original song in matter of few minutes, a similar task in a worksation may take a few hours/days depending how familiar the user is with that particular model brand.
I use this analagy: Let's say you have a transportation company, now the diference would be if:
1.- Are you transporting Rocks, sand, material contruction.
or
2.- People. It requires a very diferent type of vehicule.

You have to deside what are you going to do, and then get the apropiate tranportation.