Yeah, if they're going to hire a big name composer then they're going to want to get their money's worth and make sure they do the most prominent pieces.
No different than the voice actors really. I mean it'd be stupid to hire someone like Sheen if you don't have him play a major character, right?
Actually, I would say it's quite different than voice actors because outside of singing voices, it's usually the same actor voicing the role. Now if they were replaced in the sequel, that would be different. What we have here is an actor being replaced by another actor simply because they wanted a name actor to do the voice work for a pivotal scene, even though the original actor is going to voice the character both before and after this part.
If I was running the show, I would let the composers who were already on the team do the whole thing and not just hire some outside composer who has never done video games before score only a track and a half worth of music. I'd have a lot more faith in my composers than that.