I think all DTs should be UHF. The electrical interference present on VHF (low)frequencies and the possibility of impulse noise from lightning on VHF (high)frequencies does not go well with amplitude modulated 8VSB. Higher frequencies are less prone to noise interference when using AM. In other words less possibilities of dropouts.
In addition, VHF (low)DTs have a tendency to invade analog cable transmissions using the same frequencies. Third, I thought one of the key reasons for the transition to digital was to sell off the VHF(low), VHF (high)and high UHF 59-69 frequencies for other uses.