This is very interesting. I have an old Brother 8200 sewing/embroidery machine that I bought from a great dealer in Kentucky. We now live in Ga. Wanting new bobbins I went to the local dealer, of course wanting metal bobbins. I was promptly told I shouldn't be using metal bobbins, I should be using plastic! If that's the case, then why did the machine come right out of the box, unopened box, with metal bobbins?
So, I called the dealer I bought it from, they had never heard anything about not using metal bobbins in that machine. Now this is just my opinion since I'm still confussed over this issue, I prefer the metal bobbins. They don't seem to jump as much as plastic. I've also had some platic bobbins end up with little sharp edges that tear the thread.
Maybe it's just a personal prefernce. I'll certainly be reading to see what answers are posted.