I have used the bobbin as the top thread, but keep in mind, if you run out of thread on that bobbin you will have a mess to clean out when it gets to the end if you don't catch it on time.
I try to wind the almost gone thread onto bobbins and use them as bobbins when I'm piecing. If you run out of thread in the bobbin, there is no bunching up. It just quits sewing. Then I put in the next bobbin that has only a few yards left and use up that one. It's a good way to free up all those bobbins.