Use to be that poly thread was thought to be bad for cotton fabric. The way fabric and thread is made today that no longer is a worry. The name brand thread/fabric companies all agree on that. I use poly thread in my bobbin and love the prewound bobbins. I buy them by the case from Shoppers Rule. I also like the Superior Super Bobs and Frosties. Most of the quality bobbin threads are poly filament. Spun poly isn't as strong. Fil Tec is the manufacturer of most all the pre wound bobbins. I bought a huge cone of bobbin thread from Fil Tec and many times I use it for top and bottom piecing.
The time saved using pre wound bobbins is the main reason I use them.