When I get older machines that have bobbin thread on them, I take it off and throw it away. I've found that bobbins with thread will rust under all that thread. The bobbins that don't have thread haven't been rusty. I think that the thread absorbs moisture and you all know what water and metal do when hanging around together. And I get very frustrated when I'm sewing something and run out of bobbin all of a sudden..... and those bobbins with 5 different colors wound on them do that a lot. Also, I don't think the thread winds as evenly when there are so many pieces of thread on the same bobbin. At least when I'm taking off old bobbin thread, the deeper I go, the tighter the thread is wound.
I also use threaded bobbins (the ones I've threaded for a project) when I'm piecing quilts or practicing FMQ, etc. When I'm sewing garments I'm a color matching fanatic..... but not on quilts where they will be hidden forever.
So, I'm not a hoarder about bobbin thread. But, I think I am a hoarder about saving spools of thread. I never have to go buy thread when I'm appliqueing.... I have an infinite number of colors.... lots of it kinda old but when I pull off the outer layer of thread.... it's usually really good. In all the years I've done applique, I've only found 1 or 2 spools of thread that would break easily. Thread is really strong as long as it's stored in a cool, dry area.