The reason is simple, we are talking about one-handed Classes that all can wear a shield and a dagger to switch in via auto weapon swap. Since Dagger Finish scales with main weapon attack, you don’t even need to upgrade and/or transcend it.
Swordsman is important because it can provide 2 attack skills for every build. It also provides the one-handed sword attribute in case you want to use a sword on builds that allow it.
You can’t do that Oo. 3 weapon options allow for a lot of build options, as they are in nearly every case One-handed Sword, One-handed Spear and Rapier, which covers the vast majority of Swordsman Classes in the game.
Only Lancer, Cataphract and Doppelsoeldner are unable to benefit from these skills, that’s why I included the 3 weapon restriction cases (which are ± only restricting these 3 specific Classes) in the list.
Maybe I should make another list so you can see the difference:
One-handed Spear:
Hoplite
Retiarius
Dragoon
One-handed Sword:
Hackapella
Rapier:
Fencer
Matador
Classes that allows any of the upper weapon types for over 2/3 of their skillset:
Murmillio
Rodelero
Peltasta
Restriction-free Classes:
Barbarian
Nak Muay
Swordsman
Highlander (let’s ignore that block skill)
This means (with the exception of Hackapella since it has a riding restriction) that every spear & rapier Class has at least 7 other Class options for the build.
To make a list for a viable build pool:
One-handed spear: 9 options + Swordsman
Hoplite
Retiarius
Dragoon
Barbarian
Nak Muay
Swordsman
Highlander
Murmillio
Rodelero
Peltasta
Two-handed spear: 7 options + Swordsman
Lancer
Cataphract
Hoplite
Dragoon
Barbarian
Nak Muay
Swordsman
Highlander
One-handed Sword: 7 options + Swordsman(8 with Hackapella, but it reduces viability with riding restrict)
Hackapella
Barbarian
Nak Muay
Swordsman
Highlander
Templar
Murmillio
Rodelero
Peltasta
Two-handed sword: 5 options + Swordsman
Doppelsoeldner
Templar
Barbarian
Nak Muay
Swordsman
Highlander
Rapier: 8 options + Swordsman
Fencer
Matador
Nak Muay
Swordsman
Highlander
Templar
Murmillio
Rodelero
Peltasta
If you’re fine with being able to use only ~50% of a Classes skill tree (due to weapon restriction & mostly buffs/debuffs),
these Classes are additionally open for every base weapon:
Retiarius
Doppelsoeldner
Templar