I can understand why Rogues would be upset about all of this. I mean, sure, Shamans can wear leather. But they can also wear all of the mail pieces that drop for them. While the Rogue is praying for that one specific drop off of that one specific boss, Shaman often have a few potential upgrades available to them at any given point. Maybe it's 5 DPS lower than what their ideal leather piece is, but they could use either. If they take the leather drop now, the next boss might drop a mail item that's almost as good which will then be sharded while all of the rogues cry. In fact, this is probably the #1 cited reason for why Shamans shouldn't get to roll on leather. It seems pretty valid, too.
Looking at it from the other side of the coin, however, Enhancement Shamans feel cheated when they're barred from rolling or bidding on items that are the best possible upgrade for them in a given tier. After all, they contribute to the raid and earn their DKP just like the Rogues and Druids do. Why should they have to pass on their best equipment simply because they have inferior options? You hardly ever see Rogues choose to spend their DKP on leather items which are viewed as inferior until they absolutely have to, yet Shamans are expected to do exactly that with mail on a regular basis. You also don't see that argument flip very often when awesome maces drop that Shamans might want priority on, since Rogues could use swords instead.
Honestly, both arguments have merit, and the decision of how to handle this particular problem is going to vary widely from guild to guild. Some guilds will give leather classes priority on leather and some guilds will let whoever has the points bid on any item. There's not a good solution to the problem that will make everyone happy, all the time. A better question to ask is this: What the heck is Blizzard thinking? Clearly, the root of this whole problem is that itemization for Enhancement Shamans and Fury Warriors sucks, or else they wouldn't have to be rolling on leather. Right? Well, sort of.
It is clear that there's a definite lack of high-quality mail and plate gear for Enhancement Shammies and Fury Warriors, but there might be more method to their madness than you think. I've spent a lot of time thinking about this issue, and here's the bit of theory-crafting I came up with: The problem with hybrid classes is that they often have three very distinct sets of attribute requirements, depending on their talents, while all Rogues or all Hunters use pretty much the same attribute needs across every spec. Since raid bosses can only drop a certain number of items on each kill, an item that has many players who can use it has less chance of being sharded, and more chance of going to someone who needs it. This means that items with wider usability lead to happier player communities, in the long run.
If Blizzard created more mail items with Enhancement stats or plate items with Fury stats, those items would really only be usable by one talent spec of one class. Since raids rarely run with more than a few Enhancement Shamans or Fury Warriors, those classes would get their items very quickly and then the items would get sharded every time they dropped after that. Also, the classes that needed other drops off that boss would be upset at getting more "useless" loot. However, Blizzard has an easy solution for this problem: By letting those classes wear lower ranked armor (leather instead of mail or plate), they've drastically reduced the issue of useless items dropping. Instead of having three or four items with similar stats drop that a few classes will want, they have one item drop that many classes want. In the long run, that means that everyone gets their gear faster, even if you're at the back of the line. As for who gets what and when, they let players sort it out among themselves.
There's an argument to be made that this is a lazy way of having to design fewer high-level items, each with its own graphic (which is weak sauce), but you have to admit that the overall scheme does its job fairly well. Of course, this usually leaves the poor "off-spec" Shamans and Warriors out in the cold when it comes time to hand out gear, since many players still view leather as the domain of the leather classes. That's not a perception which is likely to change any time soon. Often, the mail and plate DPS classes will just have to accept their lot, grin, and bear it. Their turn will come, eventually.
There are two reasonably good ways to deal with all of this QQ'ing, and fortunately, Blizzard has started to pick up on one of them with The Burning Crusade expansion: Instead of having static loot drops, offer tokens that can be turned in for gear that works with your specific talent setup (although to be honest, the Shaman gear still sucks until Tier 5). This takes the "increase item usability" idea and jacks it up to 11. When a token drops, all of the classes that can use it can get upgrades from it, instead of only a specific class. Unfortunately, you can still have useless drops from this method as more and more people get their gear, and since everyone knows who can get the most use out of the turn-ins, you'll still frequently see classes ranked in priority for the drops. Another criticism of badge loot is that with the drops being so generic, it takes a lot of the excitement out of seeing what drops.
Personally, I much prefer the other method, which EverQuest II uses: Raid bosses will only drop items that someone in the raid does not already possess. If all Shamans present have the Shaman helm that drops off of a boss, then that boss simply won't drop the helm. Arguments about who gets what become a lot less problematic in this system, and you still have the fun of having random loot drops (plus nothing ever gets sharded). The counter-argument for this system is that players will burn through content significantly faster, which means bosses will have to drop less loot than they would in a random system or players will run out of stuff to do pretty quickly. Speaking as a player, I'd rather have the developers worrying about making enough content to keep me entertained than having to farm the same content for months because my gear won't drop. Sadly, their incentive structure goes the other way (it's in their best interest to keep us working on the same content for as long as possible).
For now, I'll just have to go back to waiting at the end of the line for my precious, precious leather.