I have never used poly blanket, but an old wool blanket makes excellent batting. Thin and not fluffy but warm. I think the poly blanket would be worth a try. If the quilt is beautiful as most handmade quilts are nobody will notice the batting and as long as you use it as a blanket and not a mattress, lumpiness is probably not a problem.
Store bought batting is expensive and not very warm. Plus it's just spun plastic.
If you lay fabric on top of the blanket, do you like how if feels? I imagine you can feel the pilling so that might be something to consider. As long as you quilt or tie it every few inches it shouldn't get lumpy like batting that hasn't been quilted or tied closely enough often does.
Ghislaine has a valid point - feeling the pills after you've spent time creating a beautiful top might be a let down. Also, if you are machine quilting it, I have a feeling the needle may get hung up on the pills.
Matu - you are right about the wool blanket....almost makes me wish I lived in a cold climate. Almost!
Because cotton grows so well here, I was thinking what to do with it. Now I'm thinking I could felt my own quilt batting! Some heirloom cotton from the SW is actually a pale green in color - pretty!
Subtropical desert (Köppen: BWh)
Elevation: 1090 ft Annual rainfall: 7"