Quilting stabilizes the piecing so seams don't fray. I'm thinking there probably wasn't enough quilting for this particular pattern, as it looks as if there are a lot of individual pieces in every section that were not quilted.
Quilting keeps the seam allowances from rubbing against each other. If you wash a pieced top that has not been quilted, the seam allowances will fray horribly from the cut edges rubbing against fabric.
In this case, I think 1/2" seams would have frayed just as badly. The only difference is that you would have a little more fabric left in the seam allowance.
1/4" seam allowances are traditionally preferred for quilting because they save fabric (less cost), reduce weight, and reduce the bulk that has to be quilted through.
Quilting, aside from keeping the pieced edges from rubbing, also reduces strain on the pieced seams. Seams that have been quilted down to batting and backing are not subjected to as much stress or weight when a quilt is picked up. A wet quilt hung on a clothesline would be an example of water weight stressing seams. Close quilting means that all three layers are bearing the weight of the water fairly evenly. Far-apart quilting means that some of the seams (which are weakest part of the quilt) are not tied to anything that can take some of the stress. The weight of the water can put so much stress on the seam that the fabric fibers start to pull apart.
My 2-cent theory........