while more dense quilting, especially if done with an all cotton thread or thicker poly, will be weightier, but that does NOT mean it's going to be less warm. One of my warmest quilts has a dense panto and we end up kicking it off the bed so only use in deep freeze winter. The batting is the real determinant of warm based on it's thermal properties--meaning its ability to move heat "back and forth". Poly bat has no thermal property--it doesn't breath at all, so it keeps all your body warmth in--which would be fine for really cold sleep quarters. Cotton (and cotton/poly) has good thermal qualities (less with poly in the blend). Wool has great thermal qualities.
It seems to me that regardless of whether hand or machine quilted, it should always be done to enhance the piecing. So, for example, applique may be encircled with tighter quilting to make the applique really pop; a patchwork pattern that has very complex blocks may be best quilted by a panto that does not stand out in the design, etc.
And dense quilting does not have to appear "flat"--often quilters will double the bat to give it more depth, or use wool bat on top of a cotton/poly bat, etc.