The point I would like to make is that in Cata, for example, Leather will no longer have a slight stat advantage over equal ilvl Mail/Plate. All armor types will have equivalent levels of stats (outside of Armor, obviously). The only reason for someone to roll on "incorrect armor types" in Cata is out of convenience (ilvl of current item is way below ilvl of what just dropped).

Given that convenience is the ONLY reason to roll on off-gear in Cata....Raid Leaders will need to start stepping up and telling people "NO." If DPS Cloth drops, and you've got some Warlocks, SPriests, and an Elemental Shaman competing for it, you need to tell the Elemental Shaman to cool it. If he wins that DPS Cloth, who the hell is going to make use of the Spell DPS Mail piece that drops later on? The Shaman, for the mastery bonus. The cloth wearers are still waiting.

I understand that sometimes there's itemization holes. Sometimes there just isn't a "good" Mail spell DPS belt. But it's no longer going to be a question of whether you should take a 226 cloth piece because the next best is a 213 mail piece. Now you have to overcome the inherent Mastery bonus that comes with that 213 mail piece. It has to be a much larger itemization hole.