You can't blame them for wanting to ASH to be a part of Once More With Feeling. As a matter of fact, I'm convinced that's the only reason he stayed as long as he did, and then they wrote him out in the next episode.
For me the worst thing about Season 6 can be summed up in just three words: attempted rape scene. Spike was pretty sympathetic up to that point, but maybe it was a failure on the part of the writers to suddenly come up with a really nasty motivation for him to want to go and get a soul.
No, I don't blame them for wanting him in Once More With Feeling since he had been asking for that episode since season one. But they could have left him out until "Flooded" when he returns from England the first time. It could have been mentioned that he left, and then he was informed of Willow and the Scoobies' project.
As for Spike, I wrote in another thread: "I think that's why it was good that Spike went to get his soul back. It showed people that if you need help emotionally/mentally, it's all right to seek it. That it's not going to be easy, and that you really have to stick with it if you want to see the change, maintain the change, and that at times you may want to go back, or the thought of what you did and said, or the fear of going back can be disturbing." Spike's drunken attempt to rape Buffy was something he knew (even though being a demon) was wrong and that he should have never done it. It was a hard scene to watch, and James Marsters said he had to go to therapy afterwards, but I think it brought up a good point in some relationships that sometimes people snap and while they didn't mean to hurt the other person, they can't just will it away or leave things as they. Some people seek professional help after hurting a loved one, because they realize something isn't quite right with them. Spike did that. He essentially went through "rehab". It's a completely different story as opposed to those who feel no remorse or blame their counterpart for their own actions. Spike didn't do that. Buffy deserved an ally and friend, someone who wouldn't raise their hand to her. He realized what he did and sought help. It doesn't change what he did, it raises questions as to whether or not he could be trusted, again, but it tells people that it's better to fix what's been broken and move on, but never forget.
My one thing is, people speak about Spike's attempted rape but never seem to mention when Xander tried in season one. While, yes, he was possessed, the hyena was acting on his inner feelings and urges. Drunk Spike was acting on his inner feelings and urges. The main difference is that in Xander's predicament Buffy was at full strength as opposed to being injured when Spike made his attempt. People reacted differently to it, but it doesn't change what he did. People underestimate how alcohol can lower your inhibitions, the little voice in your head not reaching you because of all the booze. It's no excuse at all. None of it. However, Spike owned up to his mistake, Xander tried to "forget it" away.