Well the colour balance on location is more or less a non-issue as your eyes compensate quite well, which is why I don't calibrate my screens differently for location shoots.
However when the screen is properly calibrated to a certain setting (e.g. 5000K , 2.2, 80cd) it's still very helpful for your clients and myself, especially for magazine photography or (if you're lucky) advertising. It's simply better than nothing ;-)
It's easier to see whether you've "got the shot" that they need/want or if there's something missing in the image or even if there's something disturbing that you yourself wouldn't be able to see when taking the photo.
I recently took some beauty shots for a magazine and they were extremely happy to be able to see all the detail on the computer (like eyelashes and makeup flakes etc) that I wasn't able to see while taking the photo. Combine that with the ability to overlay layouts while shooting tethered (which has been very helpful during an advert shoot last year) and you've got something your clients really appreciate.
And as I said I definitely need aRGB for editing photos while I'm on the road or staying somewhere.