I was using a dimmer for my uth but the problem with dimmers is that it does not adjust for the changes in temperature within the room. I would have the temp at a constant 93* and then if I change the temperature in my house, such as turning the AC up when I leave for the day, I'd come home to find the temperture on the hot side reading over 100*! this is because the temperature in the room got hotter therefore the uth is hotter because with the dimmer on uth it just keeps heating at the same rate regardless.
The thermostat will adjust the uth in accordance with the temps in the room, the room gets cold, the thermostat turns on and heats the uth, room gets hot, thermostat keeps the uth off until needed. A dimmer just keeps it on constantly.
Hope this helps!