[iano]

Unfortunately quadrotors don't scale up well. As the rotors get larger control input delay increases (because with this type of quadrotor you get your control input by rapidly changing the angular speed of the rotors). You can fix this by turning each of the 4 rotors into a conventional helicoptor rotor with collective pitch control, but at that point it makes sense to just go with a helicopter.

[Scene_Cast2]

Helicopter rotors are much more complex than just pitch control. Quadcopters fly fine with 1DoF pitch control - relatively easy and cheap to do, while helis have non-trivial 3DoF. Also, there are quadcopters with 1DoF pitch control, your comment isn't exactly clear whether you know about those.

[iano]

Yeah, sorry. Having cyclic pitch control on the 4 rotors of a quadrotor makes even less sense - then you really really might as well just be a helicopter.

There aren't a lot of quadrotors that use pitch control. That's half of their appeal; hook 4 brushless motors up to props and you're good to go. If you add in pitch control, then you either need 8 dof (4 motors, 4 pitch servos) or 5 dof (1 motor with a complicated power train to drive the 4 rotors, and 4 servos for pitch control). Both are more complicated and expensive, and again you might as well just be a helicopter.

[Scene_Cast2]

Yep, variable pitch quadcopters aren't very common, they're only used in (research-y) areas where stability is key (speeding up and slowing down propellers is slow compared to changing tilt). I agree that at this point it's approximately as expensive as a heli, just with different dynamics.

RE: Cyclic pitch on quads - I don't think anyone put cyclic pitch on quads before. I think I derived a way to make use of 13DoF for stability and dynamics that are impossible on a heli (hovering while tilting, moving back and forth without tilt, etc). PM me if you're interested in the idea, it's a little hard to describe in words.