I think there are several reasons for using a finer thread in the bobbin: you can fit more thread on the bobbin so you don't have to refill it so often, thinner thread are often cheaper so you can save some money, the finer thread is harder to see so it hides mistakes on the back, and probably some others I haven't thought of.
That said, I usually use the same thread in the top and the bobbin. I use bobbin winding time as a time to take a break and stretch my shoulders and I think that helps me quilt longer with no pain, so I don't mind winding bobbins more often. I've spent enough money on the rest of my quilt that skimping on some thread seems wrong to me. And I love having my quilting show on the back, even with all of the mistakes. So that's what works for me.....