Where the soul comes to matter is that when Buffy comes back, it's revealed that his possession of her is a stronger love than some of the loves that are actually more noble (honoring promises, wanting to be the sort of person who could be treated as a man). His loves are ranked improperly. His good loves are there, and are really good. But he wants a lesser good more than the higher good. But it's still a mistake to say that he wasn't operating out of good motivations earlier. They just aren't the whole truth about him. As he learns. And seeks to remedy with the getting of the soul.

That right there. I've tried explaining many times, and in many different ways, how I can think season 5 Spike didn't really need a soul but season 6 Spike did. And I think you just did it. :)