Great show, but there needs to be another device (carried by the quad rotors) to create very strong bonds between the parts. If the quad rotors could carry this bond-making robot that temporarily attaches to the parts and joins pieces with adhesive/nailgun/puzzle-piece fit, then they would be on to something.
Make a structure of actual use, like a tiny bridge across a moat that could hold people. The magnets have got to go.
What would be cooler would be a completely separate bond-building robot that can weld / whatever and can independently traverse the completed building. When it was too far from its next destination a quadrotor would come and give it a lift.
You might have several different types of crawler. A couple to receive materials from the quadrotor and hold them in place while a third follows a weld pattern.
Another approach would be to have a small electric lock built into the pieces. When a piece senses it is touching another piece this electric lock screws into place.
Make a structure of actual use, like a tiny bridge across a moat that could hold people. The magnets have got to go.