The situation i ran into where this would be useful was after i had subbed with Prime, later i upgraded with a 3 month paid subscription. It would have been cool if that freed up the Prime sub again to use somewhere else.

I agree. I just had jumped into a stream to check it out, and a button appeared over the stream that said "Subscribe for free". I didn't know my month of twitch prime was over yet, so I thought I was maybe gifted a sub. The moment I clicked this popup over the stream it used my twitch prime to subscribe, no warning that it would. I tried to cancel it, but you can't. Now I'm subscribed to someone I don't watch for a month, and I'm even more turned off from entering that person's stream now. I am incredibly displeased by the situation, and even more displeased there wasn't some kind of confirmation that I would be using my noncancelable twitch prime sub.