No reason at all, but
1) This would effectively be 'simulated instrument flight' due to the conditions - so would require an appropriately rated safety pilot (Rule 6)
2) Rule 17(1)(a)
Notwithstanding that the flight is being made with air traffic control clearance it
shall remain the duty of the commander of an aircraft to take all possible
measures to ensure that his aircraft does not collide with any other aircraft.
Which - if you were to fly heads in would mean you would not be complying with the above