I use them for embroidery b/c there's so much thread on them and I don't have to change bobbins as often. Bobbin thread is also thinner than regular thread so there isn't as much build up.
I buy the cardboard sided bobbins for my machines by the gross, and many times there are excellent buys. This weekend Shoppers Rule had them on sale for under $30 for 144 bobbins. If I didn't have a couple gross already, I probably would have bought more :-)