Poly cotton is fine to use in quilts. It does have a few drawbacks, which is why quilters traditionally have preferred all-cotton. Here are the ones I remember:
Poly doesn't work as well for turned-under applique because it doesn't hold a crease the way cotton does. Poly is more slippery which makes it harder to cut and sew accurately. Poly frays more easily than cotton, which means seams can eventually pull apart easier.
Also, poly doesn't fade over time the way cotton does. Old cotton quilts often have a soft look. The fabrics have faded together over time, and the fading makes all of the colors look as if they go together -- brash or garish colors tone down, etc. If you mix squares of poly with squares of cotton in a quilt, 20 years down the road it will not have the look of an antique quilt because the poly colors will stand out.