I think you are on to something----lol

I use pre filled paper sided bobbins

The bobbin for my 1917 White Rotary only have the hole in the center and a tiny one to put the thread end through. I wish modern bobbins held so much thread.

Hmmm, food for thought. I have 66's, 15's, L's and those for FW's. Must run and look for holes.
Oh, shoot, just remembered, also have two with VBS's.

I never really gave any thought to the 7 holes in my Bernina bobbins. Just always made sure that I bought Bernina bobbins for my DSM.

I have several different machines and just always figured it made the bobbins easier to identify. Never gave it much thought otherwise.

Boy, the things we can learn on this board!!! Sew on, ladies! LOL

I use prewounds most of the time. I take the paper backings off for all my machines. The paper is just there to hold the bobbin thread and keep it clean. The bobbin case does this when the paper is off.

My plastic bobbins have what "looks like" 2 holes on each side, directly opposite each other, but only one of those four holes is actually a hole. Very frustrating when starting a bobbin to find the empty hole.

I don't think it has anything to do with balance, as the bobbin does not spin fast. It only goes around as fast as the thread is used on it. Nothing needs to "balance".

Funny: I thought it was left versus right threading the bobbin.