I have a Blaze King stove with a catalytic converter, which has it's own tempermature meter for the converter. I also have a magnetic thermometer on the single wall chimney at the height as recommended by the instructions. The magnetic thermometer is ALWAYS reading below their minimum recommended burn temp, even when the stove and converter are burning hot and properly, as shown by the converter thermometer. It seems that the stove captures and disapates heat through the walls rather than having it escape and be wasted up the chimney. The stove burns very clean in this manner and the creosote that collects in the chimney over an entire winter, burning mostly dry pine, 24/7 for 5-6 months is remarkably minimal and a very dry powder for what creosote soot does collect.
So, I guess whether the chimney thermometers help or not depends on the stove being used, how it's used/burning and the stove's design. If you have a very old stove without any type of secondary burning and or if you are burning wet wood or burning any stove at too low a temperature, then the thermometer might help as a guide. But I'm not sure how accurate it would be with newer stoves burned as per manufacturers specs. It isn't accurate or needed with my Blaze Kind, but then I didn't know that for sure until I tried it out.