Like the original poster I too have always wondered why there hasn't been more development in a two bladed rotor head that is either articulated or rigid. To my simple brain it seems like it would be a good functional system - easy to hangar, cheaper to build (less blades), potentially more efficient? Maybe not cheaper than an underslung system but surely cheaper than a 3/4 blade setup? A two bladed helicopter that you couldn't mast bump sounds like a pretty good setup to me?
I see RC helicopters seem to be set up this way as well, obviously different scale of loads but surely it proves the concept?