It's probably a balancing thing. Bullets are a linear track of straight damage that already is at least the standard melee crit (3 damage for swung, 3 for pistol the weakest gun). It roughly means that a gunner can just stack bullets far more efficiently than a melee fighter or caster can stack stats to fuel their own abilities. At 3CP, it's at least equivalent to a stat point being bought at the get go. At 2CP, I can have a stack of 8 marksmanship plus an additional fire as a soldier at the start (19CP) and fire off between 33 and 98 damage over 11 to 16 shots (depending on gun type) just from that on a reset. A caster is lucky to get 20 damage a reset for 18CP (pyro-pyrokinetic being the exception). A Bound melee fighter (though without fury) would be looking at 36 damage and has to pay double cost for their next reset and will only get 18 damage for that run(assuming 6 Fire, 18CP). This is partially balanced by the fact that marksmenship, with one exception, cannot be used for status effects, but again, it just scales so much more efficiently that any other damage source even at 3 CP. Even considering the cost of another header, marksmenship stays worthwhile to seek out and buy up through the 8 point maximum (basic and expert) especially so if you pick up heavy weapons operation. Just looking at a starting build, with no list costs or weapons cost, 15CP buys you 7 shots a reset for 3 damage. I don't think you can get any much more efficient (3 to 4 CP for the skill then buying 3 additional stats costs 16 and gets you 15 damage by packet/regular melee (36 for pyro-pyrokinetic again) or 20 damage delivered from behind or 30 damage if Bound with at least 1 seal off).
As a bit of history, Marksmenship showed up first in a LARP called Endgame (which ends this year so folks should go and help them on NPC numbers!) for 1CP. As an NPC, I watched this develop into a fight set up where lieutenant monsters would cry as the damage from the guns just outstripped what would be reasonable to stat as if there happened to be no gunners on that fight. At the point where I had to stop NPCing due to time, pretty much everyone had a partial or even full stack of marksmanship just because it was that efficient for killing NPCs. NPCs had to have 7+ vitality or a called defense to even make it to melee range at that point, there were too many bullets. There was one relatively early fight I remember that lasted HOURS longer than intended because the PCs holed up and pointed all their guns at an opening in the walls. They didn't care about misses, they just fired the moment anything moved around the edge. Bosses either had to have no effect or unlimited reduce to guns or triple digit health pools that would otherwise be almost impossible to whittle down otherwise, even with the even more stat efficient Heart of Inferno-esque spells (which after 3 castings start to lose their efficiency). Calling and LE upped this to 2, but the linearity of the stat buy still made it always better to buy than equivalent stats. 3, the traditional number of "Arcane" stats for the 5-element stat systems, seems like a more reasonable level for it. It costs a little more to initially get going, but in the long run, you will have more damage than you can get otherwise. By spreading it out between lists, the header costs will round up the total CP investment and keep it from getting out of hand like it did in Endgame. Yes, you won't be spamming shots the entire fight, but you still will out damage pretty much anyone else when it is all said and done. Just pick your shots (which are significantly harder to dodge than packets) and you'll be a steady damage source that the town will love to have along.
As a note, I"m planning on playing a gunner and I"m happy with the 9 shots a reset I get out of my starting build (3F, 3 Marksmenship with machine gun assuming 4 void, 45 shots a weekend, 270 damage)... which explodes to 18 by 70 CP. A more efficient builder can start with 15 shots a reset on just 40 CP (Again 4 void with Machine Gun, so 90 shots a weekend or 540 damage)... more if you take hinderances. Admittably, I wanted a 10 shot minimum per reset when I started building to stay busy during a fight, but comparatively, I'm sitting pretty at 54 damage a reset and a full weapon style to fall back on with some melee abilities to sweeten the deal.
Edit: Now number adjusted for the 8 marksmenship max!