I guess it all depends on your definition of "useful". To me, anything I make, regardless of size, is "useful" because I enjoy the process and the fruit of my labor. When I make minis, they're useful in that they are either hung up in my home as decoration, or they are passed on to others to enjoy. Case in point, I made a quilt that was 5" square, and gave it to an elderly woman who had a miniature hobby. She had a mini castle-dollhouse, with mini furniture, and the mini quilt went on a mini bed and gave her mini hours of enjoyment. (Sorry, couldn't resist.) :)
I suppose the only non-useful quilt might be the one that sat in a closet for 40 years, but it still was useful in that (hopefully) it gave somebody joy to make it.