Tearing games apart with broken combos is fun as hell. I certainly have made it one of my life's major themes and expect to continue to do so. I just finished reading Sthenno's review of Path of Exile and he talked about the joy inherent in finding brutal, unfair combos and blowing enemies up. He disparages balance and suggests that balanced things are often less fun. I see people talk about this from time to time and to a point I agree but I think when the details are examined it becomes a more complicated than a simple declaration that balance is either good or bad.
Balance is obviously important in a PvP game. Having a single setup be the best means everybody runs that setup and people who don't just get creamed. That simply isn't enjoyable and becomes very monotonous. People of course like to advocate for their preferred style "Nerf Rock, Paper is fine. -Scissors" but really PvP games are at their best when there are many top tier strategies. This is less true in a single player game of course. I remember playing Civ 1 many years ago and it was tremendous fun figuring out what the top wonders and units were but that was fine because I wasn't playing against anybody else and the computer cheated anyway.
I think the key in either case is that there need to be terrible strategies. It doesn't need to be the case that particular abilities are always garbage but there should definitely be things you can try that are patently inferior. People love trying things out and figuring out the best ways to approach their builds and that fun is lost if there aren't mistakes to be made. If everything works fine then any random bozo can be successful and that doesn't make people feel good about their game prowess.
There are definitely places where balance is a very good thing when we look at the strategies at the top of the pile. If one strategy is simply twice as good as anything else then there is little experimentation to be done. It is fun to find such a strategy but the game quickly becomes boring after that point. When there are 10,000 strategies and 9,000 of them are garbage, 900 of them are passable, 90 of them are reasonable and 10 are superb then the game is amazing fun. There is still a sense of discovery when you finally figure out one of the great strategies but there are plenty of other things to try that don't feel weak. You still have lots of options for being bad if you want to dial up the difficulty or just explore a concept too.
The other place where balance is good is making sure that major thematic divisions are all reasonable. You don't want a game with four classes where two classes are great and two are terrible; that just results in bitterness and whining from people who chose Thief as their class. Each class should have some build that is strong, particularly if there is a big investment in a class before that becomes apparent. There is a lot more flexibility if you have twenty classes of course but I would still posit that you want some classes of each major thematic group to be good. It is boring if every melee class is garbage but fine if you can be good as an Assassin, Fighter, or Knight but terrible as a Thief, Templar, or Gladiator. At least those who want to go hit people with sticks have some kind of good option.
Balance is neither a good thing nor a bad thing in a vacuum. It is one of those things that needs to be applied in moderation and in the right way given the context.