You are wrong, think about early game - mid game - late game.
Sure late game, having 5 unique kills is trivial.
But early game? Level 1 for example, and you have a slow in your kit.
Oppressor would give you an amazing 2.5% dmg boost. right there and then.
and further increase your "early game" champion.
But say you're a mid to late game champion...
You may want bounty hunter as to better match your power spike with the power of the mastery.
it all comes down to choice.
Do you want the early 2.5% boost, to really capitalize on your early game power spike , or are you willing to sacrifice that opportunity for a chance at that 5% damage boost in the later stages of the game?.
and if you do sacrifice that, what if the enemy has oppressor and you dont?, that may have an impact on your power spike, and their ability to cope with it. Sense they have that 2.5% damage boost. they may just survive your most powerful time of the game ~ you don't want that
it all comes down to the champion, and what masteries better strengthen your power spikes.