But that's exactly why it's a Spike thing to do. Because you can't imagine him asking for it. Because it doesn't fit in the box. Because it's so insane a thought that it's exactly what Spike would do, and sure he might regret it later on, but in that moment where insanity and clarity meet he chose to get that soul, because it's what he needed to understand Buffy.

It just makes more sense to me than Spike taking a risk like that, playing with magic which he doesn't like, just to do something (aka hurt Buffy) that he's already capable of.