Definitely worth it! Do you enjoy standing in line for 60+ minutes while you watch others casually walk right on the ride in their own special line? Fast pass costs nothing, but is valuable in time saved. Just plan your time wisely. If you are at MGM, get a fast pass for either Aerosmith's RnR or ToT, whichever has the longest wait time at that moment. Then go on the other one. Go see the Beauty and the Beast show. Time will pass quickly, and then you will just walk right on the fast pass ride. Wait time with a fast pass is about 2-5 minutes, tops. You can only get one fast pass at a time though.
I think it is a great idea! Who wants to spend all their time on lines??