I'm sure you've heard this one before. Cloth gear drops in an instance, and the leather or mail wearing caster rolls Need on the item because, "it's an upgrade". Back in the day, it wasn't uncommon to see paladins wearing dresses -- and not just in the privacy of their own dressing rooms, either. It was sometimes alright to get armor that wasn't intended for your class because the stats were superior to the item you already possess. Cataclysm intends to reduce the instances of those happening by introducing bonuses with the new Mastery feature:
Along with the many gear and stat changes that are coming our way in the next expansion, we should look forward to better itemization and gear distribution. It's probably safe to assume we can finally say goodbye to other classes stealing what is rightfully "your" gear. Except for hunters, that is. Everyone knows everything is hunter loot.