We also have to remember that this could be just the tool for a newbie that is still learning the tricks of the trades.
I picked up one of these at Wal-mart yesterday for $11 and change. I mainly bought so would be easier for my mom. Easier for her to sew on lines than to mark the center line and then hope that she is getting the 1/4 or less from the line and then square up.
I can see where this could be a senseless tool in some people's minds, but not everyone grasps the other "easy" methods. And yes, I agree in theory should square up if sewing on lines, but some people can't always sew directly on lines.