Avengers have multiple powers that grant them Combat Advantage, comparable to a rogue, specific builds that are designed to milk CA, are as mobile as a rogue, and while a rogue will try to go into the enemy lines to Combat Advantage, an Avenger does it by pulling the enemy -into- friendly fire. This doesn't matter if he's ret, persuing, OR unity.
I think that assuming a rogue has combat advantage all the time just because he has sneak attack is either an erroneous assumption, or it is safe to assume a class that can apply it as easily as a rogue -will-.
EDIT: I had a party that had this same fallacy. The warlord had a choice between something that granted combat advantage and something that gave +1 to hit. One guy was saying 'Well, we don't have a rogue, so the +1 is better. My retort was '+2 is greater than +1. Seriously. Combat Advantage=good.'