It's because if you want to write a nice, happy, fluffy Buffy/Spike fic it's a lot easier if Spike wanted a soul. Less work if he wanted to be good. Same with a Willow/Spike, or Spike/Xander fic I suppose. Easier to explain their acceptance of him if he chose to be good.

I agree with you, when I saw the episode I took "give her what she deserves" to be he wanted to kill her. Although I can sort of see the intepretation that he was trying to make himself worthy of her love...which is a whole other issue because she treated him like crap for nearly two seasons but...