Brian... I don't question the need for 300-600mm lenses on 4x5 film. But the OP mentioned 600mm lenses on 135 and wondered why ultra-long lenses aren't used on LF. A 600mm lens on 135 format is equal to a 2160mm lens on 4x5 or 4320mm on 8x10. It's like shooting 6x7cm with a 1270mm lens but without a rigid body camera and with a big wind sail (bellows) attached. But using a 300mm lens on 6x7cm format is about like using a 510mm on 4x5... not too uncommon.