I'm sure others will correct me if I'm wrong, but any braking questions have been levelled more at the servo assisted braking which is designed to supplement the braking pressure of you pulling the lever, whilst the ABS monitors wheel rotation in relation to front and rear wheels to ensure the brakes don't lock.
So given that as far as I'm aware the servo-assisted braking comes as standard and this is the system some question, the ABS would simply be a worthwhile safety feature.
I never had ABS before owner my GS. Mine is the older ABS-2 system running on normal brakes (not Servo). I concentrate on braking the way I always have on other bikes and I've never needed the ABS, and am unlikely to be needed frequently, but its there so I know that should the day come when I need it, it should do its job and hopefully swing the odds of avoiding a prang in my favour (fingers-crossed).
In short, if you are happy to pay the extra for it on a new bike or its on a possible 2nd hand purchase, I would say you won't lose anything by having it and you may find one day you're glad you did...