Each weapon has it's own affinities. Look at iron baghnakhs. 70% slashing, 30% blunt. How many slashing attacks does pugilist get? 0. They also have weapons that have piercing affinity.
Gladiators have variety of swords with slashing, peircing, and blunt. one even has wind.
Marauder Bardichs have more blunt than slashing, Bjuh has more slashing than blunt, Bills have slashing blunt and piercing.
Archers have arrows with 70% projectile, 30% slashing.
Lancers have Lances with pure peircing, but halbard and spears also have slashing, and 2 or so have blunt.
So you cant say X classes weapon cant do blunt damage, or piercing damage, or whatever. Some weapons are better at doing a certain damage type than others, that is what weapon affinity is for. and that justifies the use cross-class weapon skills. I use Brandish II, Circle Slash II, skull sunder II, and Riot Blade on my pugilist, Because my weapon is better at slashing than blunt..
Marauders' Brutal Swing: "Strike with the flat of your weapon.." Swords and spears dont have flats on their weapons? Even pug weapons have flats: backhand! You can only justify archer not being able to use this skill, but every other weapon is a go.
The archer skill barrage doesnt even list a damage type does it? "Get the most out of your weapon, dealing up to four fold damage and increasing enmity". I dont see anything that says projectile damage here, yet it is learned by ranking up archer. I will damn well use it on my marauder, pugilist, and gladiator. it is something MEANT to be cross-classed. And I will tell you, this skill actually does not do a lot of damage as archer, and uses 4 arrows in the process. Tho, it doesnt do a lot of damage on other classes either, and only hits once ; ;.
Gladiator skill Rage of Halone does peircing damage. "Halone guides your weapon.." Can just stop there, as it does not specifically say sword. A Lance and a Bow/arrow are weapons too, guided by halone. I wonder if a lancer would like to have a shot at it, since their weapons have the highest piercing damage affinity. another skill that is probably MEANT to be cross classed. and this skill does a lot of damage at higher ranks, getting hit or not.
Then there is the skill Red Lotus. Deals fire damage. No swords that specifically list fire affinity, Why not invoke the power of fire with an axe or a bow too? I even use this as Mage, since they actually make better use of it.
Then there are skills like Quick Nock and Arrow Helix. Names already imply they require bows and arrows. and thus they arent cross classable.
There are also some skills that are just stupid to try to use on a certain classes. Like an Archer wouldnt want to use Circle Slash, Brandish, or aura pulse, because they still only aoe around the user, not the target, unlike wide volly.
you can say not every skill has the look and feel of using it on its main class, but does it really look the best on its main class? Take brutal swing for instance. I like how it looks when swinging a sword much more than swinging an axe. Try it sometime. You actually do do a different move than you do with your sword skills.