I disagree with your logic, Fledarmus, but agree with the sentiment. To say that disenchanted items should be universally available, but skinning and mining proceeds should not is kind of circular logic IMO. Look at it this way:
Player #1 is a miner
Player #2 is a skinner
Player #3 is an enchanter
Player #4 is a blacksmith
Player #5 is an engineer
The group gets together to do an instance. Player 1 kills a skinnable mob, and #2 skins it. Player 2 keeps said skin, and usually nobody complains. Why? Because if #2 didn't speak up or skin it, nobody would know, and thus nobody would get to profit from the leather. The same goes for mining. If at some point in the instance, #1 comes across a node, he has the option to mine it. I have never seen anyone roll for the proceeds of a mining drop. The most common reason people give is that if #1 didn't mine it, nobody would have gotten anything anyway.
However, with enchanting, it's a little different. A blue BoP item drops, and it's worth something to everyone. If someone wants it as an upgrade, it's a Need roll. If nobody wants it as a Need item, then it still has vendor value. Thus, if it sells to an NPC for 1 gold, then it's worth at least 1 gold to everyone. The enchanter comes along, and with most guild runs I've ever been involved with, the enchanter can disenchant that item to materials often worth several gold. Thus, the enchanters on guild runs are routinely asked to do that.
However, this is where my understanding and acceptance of the practice deviates from the norm. Since it has become commonly accepted as obligatory in guild runs, people tend to expect the same in PuGs. It's not mandatory in PuGs, but it is courteous to offer if you can disenchant items. However, if you already rolled for an item and won it, you won it. There should have been no second roll for the shard. It's like saying that you won something, but now that you've made it more valuable, everyone should get a second crack at it. No- it was your roll that won. Your decision to d/e it there appealed to everyone's greed, and they wanted the mats. If they want to roll for the shard, get everyone to pass on the item first, then get the enchanter to take it. Then do a random /roll and whoever wins that, wins the mats. If you all Greed first, then the winner keeps mats or BoP item. There's no "twosies" in WoW.
Also, the arguments about disenchanting mats being worth more to more people than mining or skinning is hogwash, IMO. Why? Because the argument that you can use the mats in enchanting recipes is fallacy. Yes, everyone could use enchantments. However, I could take the ore that player #1 mined in the above example and send it to a blacksmith I know to make an item at cost. I could take the rugged leather player #2 skinned and pass it on to my LW buddy for some armour. I leveled enchanting to 300 to make a profit, not give away all my goods. I don't mind rolling for an item against everyone, but if I win it, I'm sure as Hell not rerolling because they want to give the mats to someone that "wants +15 Agility to weapon", or "wants FR to cape". I want those enchants as well, and I need the materials not just to sell on the AH, but to either enchant my own gear, or enchant a guildies gear, or enchant someone's gear who is willing to pay some money for my enchants. If my efforts are going to the group, I want to see the miners and skinners roll for their goods as well. The logic is the same.