In historical times open fireplaces also served a purpose we tend to forget in this era of electricity: they provided light. Candles were expensive and not very efficient, fires on the other hand... Much better. That the heating was more inefficient than it would be in a stove or something mattered less, apparently. 
I wonder more about the fact that Skyrim has baked bread but no ovens...

I wonder more about the fact that Skyrim has baked bread but no ovens...
It also has stone grinders for the wheat, so ovens for bread should have been added somewhere.

In fact, the tapestries hung in these castles served not only as decoration, but a form of insulation (of course not an effective one).