Here in the Redwoods of NorCal it is typically 50-65 degrees 24/7 so I am usually able to go with a really light setup, sometimes just put on a down jacket and I am good to go all night.
However, if it gets much below around 47 I find myself wishing I had a bag or quilt.
I had the MBULSS#1 and sold it and bought the MBULSS#3 and it has served me very well. I too am a cold sleeper and just with it I can do around 22-24(f). If I put on my Patagonia Cap3s I can get down into the 18 range (probably a bit colder, but that is as cold has it has gotten up here in the last couple of years.)
I have, in my quest to cut grams really started to wonder how much of a bag/quilt is mental. For the fast majority of our lives we sleep under a blanket at home... but, do we really need to? How much of that is mental and an actual 'need'. I suspect after a bit of conditioning we could easy go out hiking without a bag/quilt down into the near freezing temps w/o problem. After all, the homeless and hobos do it every night, eh!