o Ventus wrote...
Sure. Except in this case, the themes presented are wildly out of touch with the rest of the story. According to Bioware, by the end of the game, the main conflict is about organics and synthetics warring with each other. How do any characters beside Legion, Tali, and EDI expand on this in any way whatsoever?
Agreed, but in my opinion a story is never really about one theme, or put another way there are various ways to interpret a story. For instance, one may argue, based on the concept of choice throughout the series, that a major theme of the series is deontological morality vs. consequential morality - or: what consequences are we as humans willing to accept for our actions? What is just war? Should war have rules?
In this way, Garrus has a lot to say during the series that is relevant.
Additionally, Shepard's status as a hero/legend is something that orbits around all the decisions you make, everyone you talk to. How does the perception of a hero influence the way the hero sees himself? In what sense do we lie to ourselves by creating idealized versions of these "heroes"? James's story is relevant here.
In another sense, you can broaden the synthetic/organic theme into a theme about the confrontation with the Other, which I think rings more true considering the entire series is about interacting with aliens. Seen this way, suddenly a lot more character arcs make sense.
The only characters that I could see as useless are ones whose character arc portrays messages unrelated to any other character/plot point in the series. Of course, it's possible that while a character is relevant to a story or theme, their existence (either through redundancy or otherwise) may bloat a story and dilute the effectiveness of its content. Are there redundant characters in Mass Effect? Probably.