B.L.E. is this your theory or are you just going by what someone has told you?

I don't know why they would install power lines if no one was going to use them. The greater the use the more current drawn, and i suspect there are many houses and commercial/industrial building being fed off of them.

As for the cancelling of magnetic fields...If the loads were perfectly balanced the fields would cancel each other out in the middle of the phases, this way the amount of 'Eddie currents' induced in the other lines is minimized. There are magnetic fields coming off the outter sides of the lines. Don't forget these lines are usually approx 100ft. and higher from the ground.