I think this is a valuable discussion point. There is nothing to indicate that The Flashing Blade is intending on experimenting, merely a desire to know "what temperature it becomes better not to use carb heat?"
A fair point that deserves an answer.
Lets use Robinson as an example. The R44s s/n 201 and prior may well be marked with a yellow arc from -15C to +5C. The later models all had a yellow arc from -19C to +3C.
Note how they could have marked them up "+3C and below" but they didn't. This comes back to how readily different types of ice form, and adhere to metal surfaces, at different temperatures.
Now lets switch to page 4-12 of the R44 Raven POH - "it is recommended that the control knob be unlatched (to activate carb heat assist) whenever the OAT is between 80F (27C) and 25F (-4C) and the difference between dew point and OAT is less than 20F (11C)"
As a pilot, it is quite simple. Apply carb heat as required to keep CAT gauge out of the yellow arc, and apply full carb heat when there is visible moisture. If your aircraft is fitted with a gauge that reads incorrectly at low power settings, apply full carb heat whenever the manifold pressure is below 18".
So if you glance down at your CAT gauge and it is reading -23C, then adding carb heat would be in contravention to the POH as you would not be using carb heat to keep the CAT gauge out the yellow arc!
If you are flying around in the extreme cold, think Canada and Alaska, then yes the thought of it being too cold may well be valid. But in the UK it is unlikely!
We should be encouraging understanding in pilots, not just telling them, "that is what we do so don't question it"
Fly safe, carb ice is a killer