Not necessary, but nice to have. I've done lots of paper piecing with regular rulers.

I use my 6" square that is marked with 1/4 inches, and never have a problem. I feel like the "add on" rulers are not necessary. All of our quilting rulers have the 1/4 inch marks and can be easily used in paper piecing.

One of the things that always impresses me about quilters is their ability to do a lot with a little. Most of the tools that are around are great and make quilting easier, but we can do without most of them. I would decide if I'm going to do a lot of pp before I spend my money on the special ruler. That is, unless, you can get it for Christmas:)