Underpowered and overpowered abilities both lead to a bland environment. Their existence means that players will feel inclined to pick abilities because of their power level being out of line, rather than how they actually want to play.
When there are several viable actions, players have a richer set of choices.
This requires appropriately balancing abilities. If a few key abilities are outliers, it doesn't make sense to change the 80% of abilities that are at a consistent power level. You nerf the 10% that are too strong, and buff the 10% that no one ever uses.
This balancing produces more "character" and "interest", not less. There's no interest in picking a skill and realizing its too weak to be a good choice, and likewise it's not particularly interesting where all characters pick the same few overpowered abilities every time either.
