It kind of depends on what style you are trying to write in.....
Arrangers (the 21st century equivalent of your mom and pop's auto accompaniment organs!) are very good at doing older styles, but often lack a good set of more contemporary sounds and styles for writing 'hipper', modern music.
But if you are writing more conventional styles and tunes, they give you a very fast way of putting entire backing tracks together, much faster than workstation keyboards (Motifs, Tritons, etc.), and allow you to experiment with different changes and rhythms far more flexibly than workstations.
I'd say, go to a store, and try out a Tyros2 or G70, or if you want hipper, urban stuff, a Korg PA1XPro. See for yourself why so many songwriters use arrangers for initial inspiration......
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An arranger is just a tool. What matters is what you build with it..!