pride listened to their fans. If fans wanted certain matchups, and both fighters were willing, it happened.
Saying that, couture DID drop and end up fighting Liddell, Franklin put on weight and went at shamrock, so it has happened before.
UFC tends to not do this now, unless there is HUGE calling for it,, But a fighter would not have to do anything if he is fighting in his best weight form.
Many fighters DO go up and down weight classes. They just "match" the weight in UFC rather than "going in raw".