Regardless of whether the album is an OST or arranged album, the arranger is the person who is responsible for how any given track sounds, and should always be credited in place of the composer. It's more accurate that way (plus it keeps my iPod nice and tidy
). Just as an example, you wouldn't credit Uematsu for the bulk of the Crisis Core soundtrack just because they're based on his compositions. You would credit Ishimoto, because he actually worked on it, whereas Uematsu wasn't even around.