There are several reasons why you might choose to wear leather or cloth armor, depending in part on what your spec is and what you expect to be doing. Those reasons include poor itemization on mail for a specific role, a lack of mail options to upgrade to in the content you're currently exploring, and simple drop luck.
Many enhancement shamans, as an example, find mail to be poorly itemized, especially as they move from five man dungeons into 10 and 25 man raiding. Since the mail designed for physical DPS is often designed for both hunters and enhancement shamans, it relies on stats such as straight attack power and agility over strength and critical strike rating which might serve a shaman better. We see this in sets such as the Desolation Battlegear, which is clearly itemized to be of use to both enhancement shamans and hunters even though hunters already have the Beast Lord Armor set itemized specifically for them. (Indeed, the hit rating bonus on Desolation is better for a lot of hit starved hunters than it is for shamans who will already have quite a lot of the hit they need from talents.) As you continue to level your enhancement shaman, you'll find that outside of Tier sets specially designed for you that you're looking at a lot of hunter mail for your DPS needs. (Now, I have to admit, I love the stats and the looks on Shaman tier gear.)
Now, in a lot of cases, these 'hunter mail' pieces will work just fine for your enhancement shaman. Compare the Mantle of the Tireless Tracker to the Tier 5 Cataclysm Shoulderplates. Similar attack power before blessings, a good deal more crit on the Mantle (and even more with the agility bonus to crit added in) vs. Cataclysm's benefit from stamina (more HP for when you pull aggro or take a cleave) and the sockets. However, it's hard to declare either of them as vastly superior to the leather Shoulderpads of the Stranger. Instead of intellect, a stat that most enhancement shamans are indifferent to, or MP/5, which the vast majority downright scorns, the Shoulderpads have expertise, a very valueable stat for melee DPS and one that's not terribly easy to get at this level of itemization. The combination of a hard to achieve stat in mail (I don't know of any mail shoulders with expertise on them, there may be some in the Sunwell that I'm not current on) with a lack of any wasted stats like int or MP/5 and a red socket makes you seriously consider these shoulders over either mail drop.
Now, for elemental shamans, it's not that caster mail is poorly itemized (since it is in effect designed specifically for you and only you) so much that it can be hard to acquire. If you look at this guide to pre-Kara drops, you'll notice that the non-PvP mail options before badges are all blue mail chestplates. Now, if you're running heroic five mans with a good group in preperation for Kara, at some point you'll probably run Heroic Magisters Terrace. I can't imagine an elemental shaman at that level of content (Magisters is hard on heroic but still doable by a talented five man group well before they set foot into Karazhan) turning down the Scarlet Sin'dorei Robes. As an elemental shaman, I might actually prefer them to the mail badge chest from the Isle of Quel'danas vendor. (In fact, I am actually aiming towards getting the Embrace of Starlight for my elemental shaman over the mail chest, which is a leather piece, but has the spell crit I want. If I decide to go cheap to save badges I may get the Blessed Elunite Coverings, which are still leather.) Less int and spell damage, but a ton of spell crit and three sockets vs. spell haste (admittedly a good stat for elemental if you're trying to get your spell rotation to include more LB's before a chain lightning) - the cloth might not be quite as good, but if you're running heroics to assemble the 100 badges anyway and it drops, why wouldn't you equip it? The armor value isn't really that important at that point, except possibly for soloing/questing where you might want to hold on to a weaker mail piece just for survivability.
But in a PuG, much less a 10 or 25 man raid, you're not supposed to be getting hit anyway. So the armor value, which a nice perk for melee DPS, means not very much at all for an elemental shaman. Now, I say this as personally still a convert to mail: I prefer mail to leather and/or cloth and will wear it whenever I feel I can get away with it, unless it's going to seriously impact my performance. I can't argue that mail is always superior for any given role, though. There's a reason that your armor proficiencies don't backwards limit you: the reason a shaman at max level can wear leather and cloth in addition to mail is because, sometimes, he or she is going to want to do so: a piece of gear will drop that is a clear and present upgrade despite being cloth or mail, and either due to loot rules being open or there being no one else who needs or wants that piece, the shaman will end up with it. I used to ignore these pieces, and as a result I've directly hurt my enhancement shaman's DPS for five mans and raids by passing on gear like the Sunrage Treads and the Shoulderpads of the Silvermoon Retainer. (On the same run! I passed to a boomkin for his offset without even thinking! He didn't even want them! If it were possible to go back in time and kick myself in the groin I would do so, especially since they refuse to drop again now.)
The point here is not to be a loot greedy jerk constantly fighting with classes who are limited to cloth or leather for their needs. The point is to be flexible and consider exactly what you do and don't need from a piece of gear: is armor value all important for the slot or are you going to see a performance upgrade for your role if you equip this item? If you answer yes, my DPS/healing viability will increase with this piece of gear, then you should never close your eyes to it simply because it's not mail. Don't let mail partisanship blind you.
Next week, I'll be listing specific leather for enhancement and cloth for elemental before Kara, and hopefully getting to restoration cloth and leather as well. I'm also hoping to do a big badge loot post for all three shaman specs.