Bounty hunter mastery doesn't see much use, does it?

It's fine the way it is, I think. Bounty Hunter is high risk-high reward, but is objectively better if you get it stacked.

I think some of the issue with Bounty Hunter comes from how difficult it is to actually get kills on more than 2 people early enough in the game. Bot can easily get 2%, but one of those laners doesn't care about dealing damage, and the other doesn't roam. The jungler needs to gank each lane at least twice (or once if they're really out of position) to stack up that mastery; and I believe it doesn't do anything if the gank turns into an assist instead of a kill for the Bounty Hunter. Mid can roam, but especially in this meta, risks losing their tower to the opposing laner. Top is the same, even if they take Teleport.

It's just easier to use Oppressor for the more reliable and easier to achieve 2.5% rather than take risks and try to go for the 5%. Not enough of a difference, and this season punishes risk-taking way too much.

Bounty Hunter is a must have on my main

Oppressor is good for mages though.

http://boards.na.leagueoflegends.com/en/c/gameplay-balance/KslZTOOW-some-masteries-need-to-be-more-appealing

I think BH is much better on champs who burst targets down like annie who are likely to get thoughs bonus stacks, but on a lot of champ this just wont happen.