I have run my Harmans that way, and my conclusion is you do not save a lot of fuel. I imagine if you use it in the shoulder seasons it would save some fuel, but in the heart of the heating system just set it to stove temp and run it.
Room temp does not save fuel either, because when the stove ramps up and down it does not run as efficiently. If efficiency is your goal - stove temp is your mode. Roomtemp is about comfort.
While I value your experience, I can't say I agree with your opinion. With that said, I'm basing my statement on using an external thermostat with the stove set to manual in the heart of the heating season (shoulder season is always hit/miss). A warm stove is an efficient stove. Look at modern high-efficiency gas furnaces. Most are now dual stage and, in some cases, some are completely variable. This is to simply burn what is necessary to maintain the required temperature without letting ducting and structure cool down too much. Heating this mass back up to achieve equilibrium is inefficient. As I said, a warm stove is an efficient stove any way you slice it.
Your first sentence suggests that some savings is gained by using a thermostat... It's also a given that you maintain a more even room temperature (comfort), so why is this not preferred? Savings + comfort is a win/win. For the record, I use a thermostat (igniter set to manual) for convenience, ease of use (for my family, less variables) and efficiency relative to stove and pellet consumption. Maintenance burn keeps the stove warm (efficient) while still putting need BTU's into the house, since we are ALWAYS losing heat. When the thermostat calls for heat, the stove ramps up to high burn, quickly and efficiently putting the necessary heat into the house to get the room temperature back up to the required temperature. This high burn is efficient (as long as no smoke is coming out of the exhaust) and helps keep the piping clean as opposed to idling or low burn all the time.
"Just set it to stove temp and forget it" - While this seems to work for you, we can't ignore the facts that other forum members will want to rely on. Stove temp will give you fairly consistent pellet usage, but that's where the consistency ends... Warm days, you will overheat the room. Sunny days you could overheat the room. Cold days you will under heat the room. Windy days, you'll under heat the room. If the goal is efficiency, then overheating is a waste. Under heating doesn't serve the intended purpose... If you end up fiddling with the temperature nob based on what's going on outside, then you are the thermostat...which isn't efficient either.
My case - Stove temp
trying to keep in the low 70's = >1 bag a day of pellet consumption, external thermostat (set and forget at 72, +) will burn <1 bag a day. That's a well insulated, 1300 square foot basement, bleeding a small amount of heat to the 1800 square feet upstairs. It's pretty consistent. Down below 15 degrees, I will approach 1 full bag per 24 hours, maybe a touch more. Either way, the room averages 72...