The only reason I can think for that is on a meta level: the Devs said from the start that Sera would be a 'love her or hate her' character and they gave the option to kick her out because of that? It's weird because I'm sure there are other characters people would want to kick out after they got to know them a bit.
As for an in game reason... no idea.
Can't see why anyone would hate her. Guess that's me...
Also, Cat, your fic was wrong, it's not Sera who enjoys BDSM, it's Bull 
You are crazy, without a voice. Sera is crazy, with a voice. Major difference. Crucial difference. Well not that much of a difference.
(not a spoiler, just completely off topic)
Are you sure I'm still voiceless...?
Only in-game reason I could probably think of is the Friends of Red Jenny not being all that useful
Bah, in game. Universe-wise, I'd rather have her Friends than whatever the rest can give me. Except for, well, The Nightingale.