I hand stitch my batting pieces with fairly large stitches(1inch)
and use double thread and stitch in a wavy line.
Stitching with a machine would make the stitches too tight, and
would possibly pull apart when the quilt is used, sat on.
If the batting isn't secured with stitches it can come apart when washed. A quilter told me about a longarm quilter that quilted a large bed size quilt for her and she only lapped the batting pieces together. After several washes and use by her teenage son the batting had separated and she had a gap down the center of the quilt.