Figured I'd ask here, someone probably has a definite answer --
I have a half basement that is really well built and has NO windows.
It really is always comfortable down there and this time of year, (midwest winter) it always is about 8 to 10 degrees warmer than the main level and that is with the heat off. (Evenings is when I look at the thermometers -- when the sun is down).
Fact is, I'm heating with a wood stove and a space heater this year (upstairs).
If a basement has no windows and well built then it IS going to an advantage to let that warmer air come up through a few registers it would seem to me, maybe even with a very small on/off blower. In fact, I have a couple of register openings I sealed of and now thinking of opening them back up.
In the winter only though -- too humid down there in the summer.
You hear a lot of people complain about their "cold" basements in winter but I bet it's because of windows and exposed foundation and I have neither.