As I understand it, the reason that PM5's OBC worked only with the Perceptual Rendering intent, is because Gretag-Macbeth did not want to mess with the colorimetric or the saturated rendering intents, which were more precisely defined in the ICC standard. The perceptual rending intent was less precisely defined, so GM felt that they could legitimately offer the OBC feature for that particular intent. This was related to me by a GM engineer at the time.
GM was a classy company, and it pained greatly me to see it absorbed by X-Rite.