This scene was one of the most important things that needed to happen. It doesn't matter how hard it is to watch, in fact for me it's not hard to watch because I know how important it is. If it had been a deleted scene so much woudn't have happened. It is what made Spike get a soul, it was the force behind a lot of what he did after.
Also, the audience needed to see this. Because Spike is a little different to some of the vampires we've seen in the fact that he is somewhat controlled by love and actually really cares for the ones he does love. Some members of the fanbase, especially Spike fans and Spuffy shippers would have been forgetting that Spike is in fact soulless. In fact, Buffy was almost forgetting that fact as was Spike himself. This event was a reminder to the fans, Buffy, Spike and even the writing team of what Spike really is. Spike was a killer, a vampire. He had no soul, was ruled by his emotions and again, even Spike says himself later that he isn't known for being a thinker. The scene made perfect sense and was very important for all of this.
So yeah. 🙂