It may turn out to be Purposely Overpowered (essentially the game designers hanging a lampshade on this trope). As an odd side effect, it will often work out that the more difficult an optional boss is, the less useful its reward will be, and vice versa. This is also often the fate of the Infinity +1 Sword (and making weaker weapons Infinity -1 Swords). The classic example is any reward you receive when you defeat the hidden, optional boss that's by far the toughest enemy in the game. It's only for bragging rights because you've already conquered the toughest of obstacles, or have already exhausted all of the game's content, making the reward mostly or wholly redundant by this point. Such a reward is typically found by defeating the most powerful monster, beating the hardest level, clearing the hardest difficulty, earning a perfect score on every level, or achieving 100% Completion.
Generally, in order to have earned the reward, you had to have proven you didn't need it in the first place. The Bragging Rights Reward is a powerful gameplay reward you can only obtain past the point where you can actually make use of it, making it Awesome, but Impractical.