I'm personally sold on the standard Marathon tires. In my part of the world, broken glass is common on road shoulders. I rarely get flats until the tread wears thin. Usually the tread will wear out on the rear tire after about 2000 miles. In my case, I see no reason to spend more for the Marathon Plus, and I don't like the fact that the Plus is so much heavier.
I did ruin one Marathon tire when I hit a broken beer bottle and got a gash that was unrepairable. I'm not sure the Plus would have helped prevent that though.