For quilting fabric a higher thread count is not always a good thing (like an 800 count!). This is why many LAQ refuse to use sheets as backing. When sewing (machine or hand) your needle is meant to slip between the warp and weft threads without breaking them. Higher thread count sheets are so tightly woven that it is impossible for the larger size needles used on most LA to slip between the weave so threads are inevitably broken This leads to weakened fabric and weakened end product. Many LAQ don't care and use them anyway and I really don't believe any study has been done to see if the quilt actually wears out faster. IMHO a quilt made with a sheet backing is a utility quilt anyway and is meant to be used to death. But if warp or weft threads are broken during quilting it could lead to some unsightly pulls on the back that you can see.
Thread count can be very misleading because it also does not take into consideration the ply of the threads and the fineness of the threads. Some long staple cotton threads can be very fine others not so much. All depends on the manufacturing quality. I won't immediately toss out a fabric that seems flimsy if it appears to be woven well and the color is perfect. I consider the application and if I can reinforce the piece with quilting. I buy 90% of my fabric on line. I have only been disappointed with the quality a few times and not so disappointed that I did not use the fabric.