I haven't ridden in an MB with 2-speed tranny, but I've ridden on tons of boats with single speed, and have never thought, "Gee, I wish this thing had two speeds."
Maybe that's why?
One benefit an inboard has always had over an i/o is simplicity, and when you start having multiple-speed tranny's, etc. you're taking away some of that advantage, imo.
On the flip slide, maybe they're on to something. I'd like to hear some opinions from those who've driven one.