Nonononono... big misconceptions here.
First of all, if Samurai were really skillchain masters, as in helping the group to facilitate more damage (which early in the game, they were looked at that way), then they would be a damage/support role. As it stands though, they don't significantly contribute to the amount of damage that OTHER party members do, so they are purely a melee DD. That also depends on what Skillchains DO. If Skillchains had other effects, like restoring HP or inflicting status (or if SAM could cause those effects), then Samurai wouldn't be pigeonholed into a purely damage dealing role.
Secondly, there can be as many roles as the gameplay invents. Claiming otherwise is like saying that all chess pieces are the same because they all serve the same role despite achieving it in different ways. Basically if you follow that argument to its natural end, there aren't six or so roles, but only two: offense and defense. Well yeah, that's kind of how combat works, so all fighting games are somewhat limited to those very generic roles.
For example, the healer and tank roles exist as a function of HP, and HP being something that can be lost and regained incrementally. The refresh role exists as a function of MP. These are not essential characteristics-- a game actually doesn't HAVE to have MP, for example, nor does it have to have HP that can be recovered. Same for TP. And there's no limit. There could be AP, BP, CP, DP, EP, FP, GP, etc., and a role for every one of those fluid or nonfluid stats. These may all be support roles in that they support offense or defense, but what's the problem with that? The point of having jobs and such isn't to cover the roles. If it were you could have as few as one or two jobs.
It's all about manipulating the channels of victory, which is what gameplay IS. And the more channels to victory you create and the more complex ways you can manipulate them, the deeper the gameplay becomes. Every gameplay element and mechanic exists as one that can be manipulated into something that an entire or partial role can center around. Is positioning a gameplay element? Then you can have a role that specializes in improving the party's position. Timing? Aim? Just working with what's in FFXI I could have easily designed twice as many roles as they currently have while making the ones that already exist more distinct. Barring PS2 limitations, of course :p
Besides mechanical roles there are also thematic roles. A Samurai and a Dark Knight have essentially the same role, but they are distinct in style. Not only visually, but in the way they fulfill their role, and that's important to players too.
Ok, now we're going to get slash fiction of Wint x Kachi somehere... rule 34 and all...
Never confuse your inference as the listener for an implication of the speaker.
Good games are subjective like good food is subjective. You're not going to seriously tell me that there's not a psychological basis for why pizza is great and lutefisk is revolting. The thing about subjectivity is that, as subjects go, humans actually have a great deal in common.