I don't care who they are, how friendly or sinister they are...I don't even care how many of them there may be. Just let them be a good, well-written character who grows and develops and is substantial in some way.
I obviously can't speak on behalf of others, but I feel that the unconscious reason why lots of people tend to react negatively to the idea of a larger group of "rivals" is because the in only game we've gotten that (X & Y) they were implemented awfully. I'll admit that, in a sense, the more characters there are, the harder it becomes to make them all meaningful. So in terms of that, I do think it may be better to have one or two rivals at most. But again, I have no preference. Well, I do have a preference, but it's just that the rival(s) is a proper, intriguing character.