Agreed, i now have around 400 hours in this game, i've tried to explore builds that isn't cookie cutter meta builds and its clear to me there is a ton of abilities (marksmanship is one of them) that is purely and simply useless. There needs to be more incentive for someone to spend 200 points into archery and put you above just regular users with a bow. At this point spending 200 points into this will not make you any better, it will merely tip the scale a little bit that with luck you have a higher chance of a weakspot proc on a hitbox the size of an acorn.
It saddens me, i really wanted to be an archer, but there is no archtype of archers in this game, everyone is an archer, but no one really is an archer, archery is simply supplmentary to melee that everyone automatically gets.
To buff this i would say either make marksmanship increase the size of the headshot hitbox allowing the marksman user to have a much easier time hitting the headshots increasing the usefulness of the 200 points spent or make the weakspot chance proc (slightly reduced chance) on chest as well so you don't have to hit the head to get a weakspot.
If you buff marksmanship i think the skill itself should only apply to foot archery, mounted archery should not get the benefit, it is already very strong. I really want to see more "foot archers" in the game but there is none for a good reason, they are absolutely useless.