The ultimate purpose of competitive game is to WIN. If some champions bring ultimately more to their team, they increase your chances of winning, combined with personal skill. Those are beaten with tactics, strategy and coordination.
Let me tell the same on an example of a fighting game. Say, one character has a gimmick that gives him ultimate advantage, dealing more damage. If a pro picks this character in a tourney and abuses it, is it good or fair? No. Should he use it? Of course he should. He aims to win afterall.
All players have different mentality, so there's no place to complain.
Sometimes OP is just OP.
This is also another reason why we need ranked dominion mode. Without ranked dominion, there is no way to differentiate between the so-called "try-hards" and the casuals. Now in ranked, Kass would be banned of course, so that's also a good thing. Then, and only then, would your argument make complete sense, because then the answer to someone complaining about playing against Kass would be "Go play ranked." He would still be OP, but you could avoid having to play against him.