For me, if I'm hunting......I'm sweating, even in cold temps.
It's also a comfort thing for me. I wear two layers up top every day of the year, just a comfort thing. Think dress shirt without a t-shirt on......just can't do that. It's the same for my hunting pants, I need a layer underneath them.
Huh, if I'm layered right my back sweats due to the pack but otherwise I don't get too sweated up unless its hot out. Esp. my legs if my pants are the right ones or vented. Also with the right pants I don't need that dual layer effect on my legs for comfort. Yes I also always wear an undershirt with a dress shirt but in turn don't have an issue wearing a single merino top hunting if that is all the temps call for. Anyways, for me the zip off base and puffy pants are handy and increase my comfort level.