Two thoughts:
First thought: we are teaching people to recognise and recover from an inadvertent stall. If that happens at low, or idle, power, then the pilot will most likely (hopefully) have the carb heat hot. Setting it to cold must, therefore, be part of the SSR, and must be taught. In the real world, pilots don't say "I might accidentally stall during this descent/approach/turn, so I'll leave the carb heat cold for the recovery". So why should we teach students to leave the carb heat cold when practicing stalls?
The second thought is type-specific, but on the Cessna 150/152/172, it is possible to set the carb heat told cold and the throttle to full power with one movement. Just use the palm of your hand to push the throttle right the way in. As the throttle gets near to being fully open, your palm will reach the carb heat knob and push that in at the same time, therefore not adding any additional time to the SSR. Granted, however, that this doesn't work on other types.
FFF
-------------