I see, I guess it depends on your local climate and if you have a thermal gradient and if you can achieve decent hot spots its ok.
Its cold here in the UK most of the time, so I get a bigger difference between the floor ant top rack. its harder to maintain a decent hot spot on all levels of a 10 level tub system for me here in the UK.
Hence my 3 thermostat system. (With the cold temperatures the floor is pretty cold and he variation is quite large from top to bottom.)
Your making me think about heating my whole snake room to closer to ideal temperatures. Or even just heating the snake room and not the racks.