Why is that necessary though? If a class requires more loot to fulfill a specific role than another class does then consider allowing them to loot more for the same price, if it doesn't impinge on the looting abilities of others. If however a class "needs" more loot just to have viable gear for different roles, I don't think it needs to be made all that cheap. When you're learning an instance, a warrior generally doesn't *need* to be simultaneously building both tanking and DPS sets. Pick one to focus on, and be resigned to not being as good at the other as people who focussed on it, or who split their priorities. When raid invites go out you're just running the gamble of whether the RL will want whatever role you looted yourself into. If you chose DPS, you're no worse off (actually still better since you have off-focus gear from previous instances) than a rogue who also lacks tanking ability.What? Yes it can. The base concept is to realize that some classes need more loot, and try to equalize the sum of what every class pays for everything they need.
Classes that need more loot... need more loot. No points system can solve that for you.
Suppose you made tanking and DPS plate half-price in naxx, because warriors *need* both. Warrior X upgrades 6 armour slots with DPS plate, and picks up 2 piece of DN. Rogue Y on the other hand picks up 8 pieces of T3 at full price, spending twice as much points. Assume for the example that Warrior/Rogue armour isn't sharing a single token, since that pretty much fucks things up in any system. THC and Gressil drop, warrior wins due to large point margin. Why? Because the warrior used the cheap costs due to his "needing more loot" to just get the loot he really wants cheap, where as the rogue doing the same role had no such option. To counteract this you then start putting in draconian rules about what loot people have to pick up and in what order, and it just breeds more discontent.
I suppose I'm twisted in that I'm used to and much prefer bidding systems, where people decide what they want the most, and bid accordingly. If they also want off-focus gear, they're willing to wait till the people who focus on it are done picking it up, and then pick it up for min-bid (i.e. practically no cost). They earned their DKP, they can spend it however they like, with the understanding that if they gear themselves into a role the raid won't need, they won't get invites.