If you build it right, there's nothing lighter that can supply the same amount of power. My 1Motor pushes 750w and weighs 4.5lbs including
battery. Not everyone wants or needs a big heavy powerful e-bike motor.
I wouldn't have bought a super light 19lb Bike Friday if I had just planned on adding 10 (likely more) pounds to motorize it. I've got a 23.5lb e bike =). So that's why a friction drive! And it works great. They're ideal for light road bikes, too.