I have to know what kind of thread it is to use it, so any questionable thread (which is all thread on old bobbins) is removed and tossed. I for sure don't want to put hundreds of hours into a quilt to use iffy thread that will fall apart upon washing. Who knows what kind of thread is on these old bobbins...polyester? cotton? rayon?
And as a previous poster replied, old bobbins that have thread on them can be rusty due to moisture absorption. That old thread needs to come off and sometimes the bobbin can be saved if the rust can be sanded off. That saying "a penny wise pound foolish" comes to mind in this regard.