Quote Originally Posted by ctrlfreq View Post
As for why, a thermostat is just an electrically controlled rheostat, and as such, it's internal circuitry is only certified to operate at a specific voltage and amperage. By under-volting that circuitry, you can potentially damage it, and leave it incapable of controlling the rheostat, leading to possible fire hazards and increased risk to the animals being heated. Beyond that, you would be trying to keep a steady temperature based on a balancing act, and a slight change on either side (ie, bump the dimmer by accident) could lead to dangerously high temperatures.
If the rheostat was going BEFORE the thermostat, I'd agree with you. However, it is AFTER, so it won't affect the amount of anything the thermostat sees.

The t-stat will be set to provide enough juice to heat the UTH to 92 degrees. If the r-stat is set to allow the passage of enough to heat it to 95, there is no problem.