You have it right of sorts. Keep in mind that most proportional controllers pulse the output in an attempt to maintain the temp of the heater when the sensor is placed on or near the heater. When you first turn the heater on, the controller delivers constant voltage to the heater just like an ON/OFF controller. If the heater ever reaches above the setpoint temp, the controller turns it OFF just like an ON/OFF controller. What makes the proportional controller different is that inbetween the low and high, it "proportions" the ON time vs OFF time with short pulses in an effort to keep the temp as constant as the controller and sensor accuracy and response time will allow. You don't get that annoying differential as with an ON/OFF controller.
Proportional - especially pulse-proportional controllers like Big Apple and GCS - keep the heating element at a more constant temp. They yield better results with under-cage style heating.
Forget wear and tear. I don't know if "wear and tear" applies to Flexwatt because the materials involved are not subject to cracking or high temps like say a ceramic element. For some heating elements though, constant warm has less of a wear and tear affect than hot-cold-hot-cold-hot-cold.
The bottom line: use proportional controllers because they give you good results, not for wear and tear issues.
