Marksmanship works well in two situations:
1) If the pilot can make multiple attacks in a round (i.e. with Gunner or Cluster Missiles), Marksmanship works for both attacks
2) If the pilot's ability requires Critical Hits (e.g. Maarek Steele), Marksmanship is the most efficient way to force these across the board.However, as far as 1) is concerned, Recon Specialist does the same thing now, so HSF builds can now afford to pick up PTL as well.
I disagree with #1 at least as it relates to gunner. Yes, you can technically make two attacks with a gunner, and this can be useful in some circumstances that are triggered by attacks. (Darth Vader and advanced targeting come to mind) But Gunner and Marksmanship don't really provide each other with sinergy, mostly because gunner only lets you hit once. Every turn that marksmanship helps you hit on the first shot is a turn that gunner does nothing. I do think that Gunner+Marksmanship will hit more and better than ether of them by themselves, but I think you get a reduced rate of return with staking these two upgrades. Remember both are pretty expensive.
Having a gunner means that I don't need to spend my action to hit consistently, so I prefer to spend my actions on other things.
One other time where marksmanship is useful is where you have extra actions, but since Marksmanship is a elite pilot upgrade you can't stack it with Push the Limit.
Marksmaship is also a pretty effective If you know you are going up against lots of TIEs, TIE Interceptors, and/or TIE bombers.