I had a GS-non-A and the main reason I chose it was ergonomics. I'm a tall strapping lad so it was a better fit than EVERYTHING else in the BMW line including the Adventure because I didn't like the extra height/weight. Getting off-pavement was merely icing, but I'd done that some on the R100R (pretty bike with street-tires!) I'd had before the GS anyway.
After a two-year hiatus from motorbikes I'm thinking about getting back on in the not-too-distant future, and another GS is the only bike I dream about.