Holy unintended consequences, Batman!
In addition to being totally unworkable to to the human engineering problems Lusche and Hitech have addressed, turning on friendly collisions is absolutely, completely, 100% unworkable without the imposition of some sort of traffic control at friendly fields. And who's going to do that?
Think about it, what would the daily carnage at any RL airfield be if there were NO tower or air traffic controllers? And that's with professional pilots. Even if you turned it off for planes with wheels on the runway there'd be constant collisions taking off and lining up for landing.