Usually.. "rivalries" bring unpredictability. Like Fed-Hewitt or Fed-Roddick, or even Nadal-Fed usually brought NO unpredictability.. You damn well knew who was going to win the majority of those matches.
Slower surfaces like clay, slow hard courts, or slow clay, Nadal was going to have the advantage over Roger 8 or 9 times out of 10.. . Indoors, Fed was going to have the advantage 9 times out of 10.. And thats how it played out. Or if it was just a best 5, Nadal usually got the advantage while Fed had a better chance if it was just a best of 3 set match.
Fed-Roddick for instance.. THAT was a rivalry? You knew Roddick would flub it up if he was in a position to win and Roger would get the best out of Roddick at the end.. Again.. TOTALLY PREDICTABLE. And the h2h kind of showed that.. Thats not a rivalry IMO.. Because Roddick could never beat Roger even on Roger's worst day..
I don't know if Rivalries should be considered "bad" or the "worst". If it was a true rivalry, you wouldn't know who the heck is winning.
Best of 5, slam time, Nadal-Fed.. You put your money on Nadal damn near EVERY TIME as it played out the same every time. (Fed starts out strong, but Rafa continues to impose his will and Fed faulters. It was a reoccurring theme through their entire careers). Fed-Roddick ANY TIME for instance, you don't even put a penny on Roddick.
Rivalries shouldn't be overly one-sided and predictable