I get asked the question which is at the top of this post a lot. Most often it's when I teach my Buttonhole Boot Camp class. The answer is "no", I always use thread. I do this mainly because I spent so much time sewing it wasn't something I originally thought to do in any other way. Now that you've asked I do have some reasons.
Thread is often stronger than yarn.
I don't want to sew in more ends of yarn. I know some knitters use a single strand weaving it from button to button. I don't like to do that on the back of a button band because it can be seen sometimes and it interferes with the stretching of the band.
Yarn is often too thick to go through shanks and buttonholes. Many knitters split the plys to fit, but then it is weaker.
A slippery yarn may not stay knotted.
Yarn can stretch, thread is more stable.
I can create a thread shank to make sure my button floats above the knitting without distorting the band. Thread has more body than some yarns.
