At last Saturday's guild meeting, I sat next to a woman that was using the Play on the Fly technique with her spindle. She remarked that this was the first time she'd seen me bring a wheel to the meeting. I usually bring a spindle. So much easier. The timing was great though as I had been thinking about playing with that technique and asked her to show me what she was doing. It has been a couple years since I was introduced to it and never actually got a comfortable rhythm using it. Over the week, I've experimented. I now feel comfortable using it.
My thoughts are this..
It is great for sampling a three ply yarn. If you use an Andean ply or chain ply for your three ply, this is another option. Because your spindle size limits the amount of yarn that you can produce for one skein (or one cop), it isn't particularly good for my use. I spindle to make enough yarn for a project. I want good size skeins. If I make 3 cops and ply them together, I can produce a 200 yd., 3 ply skein. Using this technique, I'd have 3 skeins of about 67 yds. Something to consider when you are making a project. How many joins do you want to deal with. I played mostly with silk and if you knit with silk, you already know that it really requires a Russian or sewn join. That alone would make my decision.
I'll use this, as I said, when I want a fast three ply sample. Give it a try. There are several good You Tube tutorials.