As I've said before, I'm in favor of not allowing stacking of advantage/disadvantage. I don't want to encourage at the table the hunt for additional sources of advantage/disadvantage just to put you over the top. It much like the annoyance of players hunting for +2's in 3e, with a more noticeable game effect. A binary method is quick and simple, and doesn't encourage this. Maybe as a module/option, but I'd rather it not in core.
basically, you discourage creative thinking with that.