Here a short list of things you can ONLY do with a CS in arranger mode...

Play another instrument.
Play another keyboard.
Solo using the bender like you would if you DIDN'T have to play the chords.
Solo and use the synth knobs or sliders to mess with the sound.
Play the head with simple chords, then play full piano and get REALLY 'outside' without screwing up the chord recognition.
Set it to vamp, and go out into the crowd.
Scratch your arse, while STILL playing 'Peaches en Regalia'!

I've always said, generally it is a feature for the better player. If all you can do right now is simple chords and a simple melody, it doesn't make much sense (but it COULD help you develop a lot more skills...). But if you have a very strong LH (pretty much wasted inputting the same rote chords over and over again), or well developed soloing skills using the pitch and mod wheels, or want to try more outside stuff that the chord recognition isn't happy dealing with, it can open some pretty big doors.

Don't forget, the CS can be anything from a two chord vamp, to the entire verse and chorus of a song, to the whole darn thing (especially if your arranger allows the CS to be saved, and loaded)!

I am actually having more fun playing in arranger mode than I have had for ten years. Now I have my CS back, I am not quite as tempted to make an SMF of the arranger doing the backing for a song, so I can play with TWO hands! I can solo as strongly in arranger mode as I can over an SMF...

It isn't a MUST HAVE... But you sure miss it when you don't have it, if you have the skills to use it.