If I'm not mistaken the plastic bobbins came about when they started making the low bobbin indicators on sewing machines. The mechanics of it was that the low bobbin indicator could not read through a metal bobbin to let the user know of a low bobbin. That is what I was told by Pfaff. There are any number of higher speed machines that do fine with the plastic bobbins. They are engineered that way and are not really made of what we think is plastic...it is another type of high grade material that has no problems with high speed. I think there is a lot of misinformation out there about bobbins.
That said, if your manual says use only XYZ bobbin then of course use only that type of bobbin but a I don't think a plastic type vs metal type class 15 will make that much of a difference. Both types can warp with a lot of use. Also some newer machines aren't made with the all metal bobbin cases and the metal bobbin may be too hard on them. I am speaking of the less expensive machines that are available today. However the higher end machines should not have a problem with either unless you have a low bobbin indicator and want it to work properly...then you need to use a plastic type bobbin for your specific machine. Again, these bobbins are NOT plastic but some new high grade type material that were created specifically for these machines.