Individual classes don't need unique spells in my opinion... but rather individual characters need unique spells within a party.
A bard that has Thunderwave is unique if none of the other PCs have it or prepare it. And this is where working with your fellow players to create a cohesive unit comes into play.
If a Sorcerer is deciding to make a pyromancer, I would hope that the Wizard would choose a different spell selection that doesn't include spells like Burning Hands and Fireball so as to differentiate the characters, both mechanically and narratively.
A bard that has Thunderwave is unique if none of the other PCs have it or prepare it. And this is where working with your fellow players to create a cohesive unit comes into play.
If a Sorcerer is deciding to make a pyromancer, I would hope that the Wizard would choose a different spell selection that doesn't include spells like Burning Hands and Fireball so as to differentiate the characters, both mechanically and narratively.