Post-BB Bowmaster SP Distribution

Thanks, I'll remove that note. =)

Marksmanship is taken because you do more damage to monster, if I understand the skill correctly. For example if you have a monster with PDRate 20, you will do 20% less damage to it. If you max Marksmanship, the PDRate is lowered to 0. So your damage goes from 80% to 100%, a boost of 25% (percent, not percentpoints).
On monsters with for example PDRate 5, you go from 95% to 100% (5.3% boost), and putting more then 3 points in Marksmanship won't change anything with your damage. On monsters with higher PDRate, for example Pink Bean has PDRate 70, you go from 30% to 50%, a boost of 66.7%, a massive boost compared to your other skills.
The keyword is if I understand the skill correctly, I'm not sure about it.

And you won't train faster if you die every time because you didn't spend points in Marksmanship.