I've been out of it for a while[retired in 2010. worked for Northeast Utilities now Eversource] did 20 years in transmission mostly maintenance 115& 345 and another 20 in the district doing distribution. I believe the main reason that type of ROW clearance isn't used it's cost, most ROW are the cheapest roughest property available lot of swamp or rocky terrain so getting there and securing it with a fence is troublesome and expensive, Because it is rough country you have to worry about wolves coyotes bobcats fisher cats any number of critters that will impact the animals. I remember in the 70s on a ROW in New Hampshire they had a herd of goats that cleaned a ROW but it was only used a couple of years. As far as EMF I do know that we never allowed any buildings or sheds under our lines I know it was in case the line fell but I also know that there was a lawsuit from a farmer whose cows stopped producing because of stray voltage on a water pipe feeding a water trough.