Makes total sense to me, it's just easier to work around an established character with (some) characterization set in stone and I certainly wouldn't put that as one of Mass Effects faults, if anything I think Mass Effect did so well in general because of just that.
It's a matter of opinion, but I do think it's a fault. I'm mostly angry at the trend it started than the game itself. It vastly limits the character and dialogue options can end up pretty vague or be a completely different idea of how you think a normal person would handle something and how a nutjob writer thinks a situation should be handled. It also tends to railroad you more often with it being more obvious the choice is an illusion.
It can vary greatly between games - albeit a few options in Mass Effect had me scratching my head - and I'll never forgive Bioware for it. Never!
I think you're misrepresenting the comparison somewhat.
Not really, it's how the forums itself sees it. The Champion is the writer's mouth piece. It's why your champion comes off schizophrenic between any major characters or even minor. Because each writer has their own idea of what the champion is. There is no cohesion, no coherence. Nadda. A good example is how a writer will write the champion as confident and sure, but then another will write him as treating himself as a punching bag without even choosing that as an option. The writers have default attitudes and they clash constantly.
This wouldn't be as much of a problem if the Champion was considered the player's character. Since it's not, then they give their default idea of the Champion - leading to clashes - rather than put in more neutral responses until a decision is made to guide the narrative as interested or disinterested.
Mass Effect didn't have this issue because they made sure the team understood the basic core of Shepard. So either they're ignoring the codex - at which point why have it? - or there is not much of an established core.