This, @Imrix. Not all MAs are equal, not all MAs have the same uses. Buying up Martial Arts once and applying it to every Style is pretty much analogous to buying up Dawn Caste 5 and apply that as your prereqs for Dawn Abilities.Martial Arts are self-contained yet cross-compatible toolkits that encompass more than one vector of combat. A character who invests 20 Charms in Brawl will have an extremely powerful offense, but will require investment in a second, different ability in order to obtain any defensive capability. A character who instes 10 Charms in Brawl and 10 Charms in Dodge is covering offense and one of his vectors of defense, and is paying two different abilities to do so. Similarly, a martial artist whose investment covers 20 Charms - about two complete styles - will cover both offense and defense, and pay two abilities for that price. He will in fact pay more (the Merit) because the right choices of MA can cover multiple vectors and styles of attack and defense (like say, both Parry and soak), and the new rules on Form Charms make it convenient to swap strategies mid-fight.
I'd also like to note this:
You are also not necessarily meant to, you know, get perfectly even returns on mastering five different martial arts. I'm pretty sure someone who masters Single Point and Crane Style and Black Claw and Righteous Devil aren't necessarily gonna have the same sort of synergy and power as the guy who mastered Snake and Tiger Style.. You spent more experience mastering four styles, sure, but you weren't picking styles that complement each other, so you have a wide tool kit but not necessarily one that works super well together.