It would be convenient to pick someone who found faith because they were dying or had nothing left to lose.
I think a lot of the fic writers were trying to get rid of the pesky consent issues (much like the pressing assumption is now that Jimmy died in 4.22) by putting the vessel at the end of the journey.
I think Kripke or Edlund or whoever definitely made the more interesting choice in having Joe Normal (+faith) be the one instead.
But they didn't have to worry about porn. Well, not on the surface, anyway.
What made Jimmy's kid say yes?
Love? Not knowing better?
Demons without hosts and Anna with her called in favours were definitely things dropped. I think they cheated with Anna in order to get actress continuity, which I can't totally blame them for, but when so much of the show is about vessels... she was able to evince much power in her constructed meatsuit, so it falls on the storytellers to explain to us just why this was never a solution someone as powerful as Lucifer or Michael could exercise.
And the demon thing with Bobby, I'm guessing they just hadn't started thinking yet. Oops.
Does anyone know if Kripke had already decided what Mary was apologising for in Home?