I have some metal bobbins and some plastic. I much prefer the metal ons.
However, i do use the plastic ones for a specific purpose. I always use my "good" thread for piecing and quilting. I always put my "good" thread on metal bobbins. That way I know it it the good stuff!
I have a bunch of older, cheaper thread that I use for mending, basting, or making something that I know is not going to be expected to last a lifetime. I put that thread on the plastic bobbins. That way, I know it is the less than great quality thread! Works for me!