Considering that I just finished a quilt with a tissue lamé binding, I guess I am not a purist. <VBG>
I have used a lot of polyester thread, and probably will still use it as it works, because I have a lot. But it is good quality poly thread. Have mixed some poly-blend fabrics in with cotton (but I also mix pre-washed and not washed fabrics in the same project on occasion.)
Never again will I try to applique with poly-cotton fabric though. Talk about difficult and fraying.
Here in the desert I have always preferred to sew with natural fibers. But blends were most abundant for a while.
If it works, it works.
Have fun,
Pati, in Phx