I'll follow this thread with interest since I've thought of installing such a cover on my awning, too. In my opinion, the biggest benefit would be protection from UV radiation for that last wrap that's exposed to the weather all the time.
I've read many "replacing fabric" threads where the only thing wrong with the original is that those last few (exposed) inches at the rigrail have dried out, cracked, and/or torn. Seems like a cover like the one you describe might prolong the life of the fabric by quite a bit.