Marksman or Bowmaster

The main reason people say they lose less, is because once you can 100% 2hko a Skelegon with Strafe, you've reached your limit. Bloodmastery makes that take longer for Xbowmen, but they still hit that limit. Bowmasters on the other hand would still be able to reduce how many Hurricane arrows a Skelegon takes.

If you boss, then you lose the same amount of damage proportionally.

It can still have an effect. I never passed stats, so I won't go into percentages. But one of the things being (slightly) stronger can affect is if/how often you can OHKO with 4 crits, how soon you can OHKO with 3 crits, how many total crits you need within two strafes to KO; do you see where I'm going with this? Every little bit of damage helps. Yeah, not as much as a Bowmaster, but it's definitely not as insignificant as you make it out to be.