The place where that theory always trips up for me (soul as moral compass) is the way both BtVS and AtS sort of duck the issue of human evil. Such as the Trio in S6, or Willow - humans with souls making bad choices. Or Buffy herself, for that matter. So it was always hard for me to see how putting a soul in Spike "fixed" him in any way. The average moral compass isn't all that convincing even in the "hero" characters in the Buffyverse. Especially when sometimes the soul issues tends to come off as "no matter how these characters behave, objectively speaking, there's this invisible 'soul' thing which justifies me to treat them as second-class citizens, because eventually they will let you down." That smacks of self-fulfilling prophecy.

I guess that's what I meant by the soul itself seeming like a bit of BS in aggregate. Buffy feels it's okay to kill Anya (as a demon) but cuts slack to Spike... questions of conscience seem extremely variable and personal.