Good question. My first response was that everyone needs enchants, and everyone can use those mats. But if everyone can get their friendly guild enchanter to enchant something, can't you get your Leatherworker to use skins, or your Blacksmith to use ores? Of course, you could argue that Leatherworkers can get skins from anywhere, but disenchanted blues only show up in instances. If it's an item that required five (or even 25) people to get, everyone should have a chance at it. There are herbs and ores in instances, true, but those can be found elsewhere as well -- they don't need a group to get them. And what about Rogues who unlock chests in instances -- sure, we need them to open the chests, but they need us to get them there.
You can get blues through questing and drops, though, too, so who knows who deserves what. Sardonis is at the point where he won't even say he's a disenchanter -- he'll just do a greed roll like everyone else, and if he gets the item, then he'll DE it. The tradition seems to be that we all roll when we've all helped drop some boss loot, but it's true that we'd never get the shards if it weren't for DE'ers. Maybe they do deserve to take what they make.