Having watched many of the anime you listed i still do not recall Back stabbing assassin Samurai in them. Though the idea of a Samurai Tank gap closer being a dash through with a 1-2 second stun to turn the enemy away for DPS positionals sounds rather cool.
As for your point on Pikemen , yes that is a valid point well made. I have always felt that since the only Dragoons in historical existence were the mounted cavalry force, I believed the idea was more of a polearm specialist.
Something more in the vain of this.
https://www.youtube.com/watch?v=lS7sExRrCbw
An Anime reference should the later not be sufficient. (Spoiler Warning: These scenes are of duels featuring spear fighting in the Anime series - Seirei No Moribito, as each fight is concluded, story elements are reveled)
https://www.youtube.com/watch?v=al1CpCfSEHo
(Source:http://finalfantasy.wikia.com/wiki/Dark_Knight)
Dark knights, also known as black knights, were a minority group of knights who refused to serve lords or kings. Instead they either looted and raided, hunted bounties, or lived off the charity from the more humble folk, in much the same manner as a Japanese ronin. As they had no master, and thus no squire, they would paint their armor a signature black color to prevent rust and damage. Dark knights were considered by many a necessary evil, as they would protect villages and lesser people from invaders and bandits, but would also cause problems in the regions they dwelt in to sustain themselves.
Back on topic however the idea of the Samurai you have explained has always been when the Hero is facing a small mook army of no comparable skill. At 60 I can walk out in a low level zone and "kill (my) enemies in a single blow 10 v 1 and everyone standing nearby is shocked and amazed at (my) l33t skillz0rs". That's hardly justification to identify the role of a melee class/job.
Ninjas fought through guerrilla tactics specifically to turn the tables on their Heavily Armored Samurai opponents often not fighting but only gathering information. Monks used their training not only of their bodies but their knowledge of anatomy to strike precise points of their enemies form any and all angles. Dragoons are a mounted cavalry unit in the British armed forces.