A lot of guilds restrict loot by class. They justify this with the phrase "Guild progress comes before personal progress."
As a counter, I offer these words:
"By pursuing his own interest he frequently promotes that of the society more effectually than when he really intends to promote it. I have never known much good done by those who affected to trade for the public good."
- Adam Smith, An Inquiry into the Nature and Causes of the Wealth of Nations, 1776
Liberal capitalism is, in general, the dominant economic system in modern times. It's fundamental idea is that of the Invisible Hand: that an individual seeking to improve his own situation will unintentionally work for the benefit of the group. For the most part, the system works in real life.
The same philosophy should also work in WoW. A neutral system of earning and spending DKP should lead to people seeking to improve their characters as they want to, and thus unintentionally improving the raid. I have not yet seen a good reason why the Invisible Hand does not apply to WoW.
The argument against this is "a warrior will do more dps with Uber-2H-sword than a paladin on a raid." And this is strictly true. So how then does giving the paladin the Uber-2H-sword improve the raid? For one thing, the paladin will be less likely to quit. If you are prevented from improving your character as you want to, your enthusiasm wanes. If the paladin quits, the guild loses multiple epics and a skilled player. This is a bigger loss to the raid than a single weapon being used slightly less efficiently. It's generally taken as a given that healing classes, in addition to being the rarest, are the ones which burn out fastest. Keeping healers from burning out, at the cost of one or two dps epics going to a healer, improves the raid.
As well, people do other things than raids. They quest, farm and PvP. By making a rational decision to improve some of these other areas, they benefit the guild. As a personal example, I'm currently working on farming for my Hide of the Wild. I'm specced for raiding (30/21/0 - raid gets Blessing of Sanctuary), and thus farming is painfully slow. A good epic weapon would make farming easier, leading to a better healing cape which would improve the raid. In fact, farming is so painful--and I have to farm for some other enchants--that I am considering to switching to 20/0/31, which means no more Blessing of Sanctuary and worse healing on raids.
In the end, people are most productive when they are improving their character as they see fit. A raiding guild should think long and hard before abandoning the wisdom of Adam Smith.