I did miss what you intended. I get what you are saying but you have made two assumptions. Your scenario is great if you are using a on off t-stat and if your room temp is reasonably stable.
A proportional t-stat feathers the power and usually does not deliver 100% power or at least only rarely. The second issue is if the ambient air temps drop the rheostat will limit the max heat and if that temp is not enough to over come the rooms ambient temp the hot spot gets too cold. Racks don't do well in cool rooms so that isn't a big issue with a rack but when applied to an enclosure it can be.
Your car analogy is great but if using a proportional t-stat it is more like an accelerator (t-stat) and a governor (rheostat) restricting the flow of gas to the engine. That is great until you need just a bit more speed than normal and the rheostat will not allow it.
The new generation of t-stats are very 'smart' they are able to shut down power if they detect over temps but also under heating too. So if the probe gets pulled away from the heat source. (very real possibility with a rack) they add an extra bit of safety margin. I still recommend a dual t-stat set up especially in cool environments it allows the primary to do its job but if it fails or something happens the secondary will shut down the power to the primary.
The numbers are my own numbers and variable but this is the basic idea. Either way we are saying the same thing a T-stat should have a secondary safety system. A t-stat should have something to else in the advent of catastrophic failure.
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