The 'logic' behind it is this: Sera is 'worse' than Vivienne, so you aren't allowed to criticize Vivienne because she is not as 'bad' as Sera.
I think it's a form of the 'appeal to worse problems' fallacy. Vivienne is 'bad' but Sera is 'worse' so you aren't allowed to say Vivienne is 'bad'.
But I've never seen anyone offer a cogent, rational, argument as to why Sera is the worse companion beyond idiotic things like: "She's snotty (so's Viv), She teases everyone (Viv does this too), I don't understand her/her accent is horrible!" Occasionally someone will offer up the noble killing but that's easily countered by what Blackwall has done or what Leliana might do depending on how your game plays out.
I've always felt Viv is a bit worse given she comes from a position of power and influence already, before even joining up. Sera is at best a street kid nuisance with good intentions.
But the bottomline for me is I would rather have all of these characters in game warts and all. Who wants a bunch of do-goody-good-glee club members who wouldn't be caught dead taking a ****** in an alley? I am fine with all of these characters as they are although I think the decision to make Sera bootable still needs an explanation because that has greatly contributed to this state of affairs.