It kind of surprises me that this is even being debated. Let's start with the basics. Is it fair? Of course. Every person who plays the game has the "ability" to juggle, it's just a matter of whether or not they actually know how to do it. Juggling doesn't require a special controller to do. It doesn't require any mods or hacks. It's something that is possible through normal gameplay, thus rendering it a fair tactic.
Moving on from there, could juggling be considered "cheap?" Not really. Juggling requires a lot of skill, and typically something that is considered to be "cheap" does not.
Juggling, IMO, adds something very important to CC. It adds a skill gap. For anyone here who plays games competitively, you know that a high skill gap is incredibly important. Skill gap, for those of you who don't know, is the difference of "skill" between the worst players and the best players. Games like CC have a HUGE skill gap because of juggling. A new player has basically no chance of beating a player who can juggle well. That, from the eyes of a competitive gamer, is a good thing.
However, I'll admit that juggling hurts the CC multiplayer community. New players don't want to play a game where they get dominated without a fight. Honestly, the arena would be better if there was no juggling because more people would play. That being said, I personally enjoy juggling because it widens the skill gap.