I think that traditionally hand-pieced seams were pressed to one side to strengthen the seam. This convention was continued when quilters started machine piecing; however, it's not really necessary with machine-pieced seams because they are generally stronger than hand-stitched seams. Many award-winning quilters press their seams open to achieve more precise piecing results and to make hand quilting easier.
"Bearding" of batting through seams pressed open was probably more often a problem in the past. It might still be a problem today with some polyester and wool batts, but I think for the most part modern processing of batting has cut down a lot on this problem. Cotton batts usually do not beard at all.