When flue-curing, it is green leaf going in the kiln, lots of moisture coming off the leaf in the curing process - needs to be vented while maintaining proper RH, with an off-set lid on the crockpot. With a wet-crockpot adding moisture, and some ventilation, I avoid any leaf from premature green-drying spots.
My experience, not all the leaves loses their green uniformly, so, while checking the kiln a couple times a day, I'll slowly raise heat and cover the crockpot to reduce the RH, and keep the ventilation going, until about 100% all yellow/brown but still in decent case. Then getting to the leaf drying phase (and stem drying), my crockpot is running empty of water, but wide-open ventilation (as much as can be expected with just 1/2-inch exhaust pipe). It usually takes me 2-3 days to go thru the leaf & stem drying phases while constantly increasing the heat. When flue-cured is finished, I am back to a wet crockpot to bring back the tobacco to a good case to handle & store my finished tobacco. This works for me !
When kiln fermenting (air/sun-cured burleys, turkish, cigar-tobac, whatever), the temperate & RH need to be held constant more or less throughout the weeks long process. This works best for my kiln by having a very slightly off-set lid on the wet crockpot, and allowing some ventilation. My 1/2-inch exhaust valve has a ball-valve that I'll partially close as needed. It's a balance between heating & venting to keep it constant. Plus, during this fermenting process, I am off-gassing the ammonia and whatever else - It is that beautiful aroma coming out that tells me it's working.
I have a thermostat control on the crockpot, set for temps between 122 & 128 degrees for fermentation. It is usually a couple hours going thru each on/off cycle. I don't think it's "a waste of heat and humidity", it's what it takes to maintain that temp/RH balance.
I have 3 small 'computer' fans in my kiln, to circulate air throughout, only 'on' when crockpot is on - not sure they're needed, but I feel it helps keep that temp/RH constant top-to-bottom of the kiln.
Remember, it's only a itsy-bitsy 1/2-inch exhaust pipe/valve, fed by thermal currents (hot-air-rising), cannot be that wasteful.