This is just my thinking with nothing to back up my thinking.
The heart muscle does not act like a leg muscle when exercised. The heart can take much more exercise than a leg muscle with not bad side affects. So if the heart rate goes up it is not as physically stressful on you as having to pedal in a large gear (mashing) up a hill. Also a higher heart rate is moving more blood through your system. The increased flow rate helps to get rid of the waste and toxins in the system while supplying more food and oxygen for the muscles.
So it seems to me that low leg load (spinning) with a high heart rate and slightly labored breathing is the best way to ride for endurance. I also think that endurance is the key to losing weight.
Like I said I don't have any data to back up my feelings so feel free to not take my comments as being correct.