You really do have a good point.
This is just my own opinion, of course, but under normal circumstances, you only need the extra heat before fermentation gets really underway and again when it has slowed enough that fermentation is no longer generating enough heat on its own to keep the temperature where you want it. That would typically be only in the winter time (for me).
I can't say that a brew belt, which is not even thermostatically controlled, would be much safer. It continuously generates heat at the same rate, regardless of the surrounding temperature.
With a brew belt, depending on the climate, one has to turn the belt off during heavy fermentation, then back on again later. In my area, if I should leave the belt going during heavy fermentation, my temperature would go much too high for me. My basement stays at a pretty continuous 66 degrees F.
But honestly, because it is not that cold in my area, I use brew belt(s). I have two identical belts; one gets much hotter than the other.
I am sure a person could make his own heater out of much more reliable parts. If I needed more heat than I typically do, I just might take that project on, myself. I have read where others have built sophisticated controls that have a heating probe that that goes down into the must. Now that's nice!!!
