Hi Travis, I don't think you're missing anything.
I do use a sleeping bag liner, I have a Sea-to-summit Reactor (+14F). I mainly use to keep my bag clean. After hiking and sweating, and after a few days in the field, I qt least know my sleeping bag will require less care.
I am a hit sleeper, so I always sleep in boxer in my Magma 15 sleeping bag up to 27F ish with the liner. It adds a little weight, but I believe it extends the life of of sleeping bag, or at least minimize the cleaning needs.
Curious to hear the others on that one.