I have used a bobbin as top thread a few times, but only if the color I really want is already on a bobbin. Have you ever run out of thread from the top thread? It's a real mess.
If you run out of thread in the bobbin where it belongs, it's just the end. Put in a new bobbin and you're good to go.
If you run out of top thread while sewing, it's a mess and you have to untangle it and get rid of all that excess thread mucking up your seam.