I'm beginning to feel sorry for the Add a Quarter ruler! If you don't want to use it, then don't. Of course the sellers of the ruler make money - that's why they are in business. But they also make things easier for those of us who choose to use their tools. If I was giving a class on paper piecing, I would absolutely put an Add a Quarter ruler on the list. Why not make it easier for the participants in the class?
When you're first learning paper piecing it's very easy to forget and trim your fabric on the line, rather than 1/4" from the line. The ruler is for trimming fabric before you sew the seam, so you can line up the next piece, not after the fact. If you're using it to trim after you sew a seam, then you aren't using it for its intended purpose and it wouldn't be useful. Use it as intended and it is useful.
Show the Add a Quarter some love!