Obstruce's aunt disagrees, especially with Shamans and Druids -- if for some reason they pull aggro, wearing leather or cloth will only make things harder on the group. Which is true -- if I'm wearing leather (or even cloth) on my Shaman, I'm not going to have near as much armor as I would wearing the mail I'm supposed to wear. But in a group where all members are doing what they should be, I should never get hit anyway. If a healer's getting hit, it's a good 80% of the time not their fault -- it's the tank's or DPS' fault for not keeping aggro where it belongs.
So no, I don't see any problem with a Druid or Shammy (or even a Paladin, though there's a lot of nice healing plate out there anyway) wearing less than they're meant to. Warriors are definitely not in the same situation -- while yes, some of that Hunter mail may have lots of Agility on it, and that will help out your crit percentage, you get so much more bonus from Strength and Armor that it's just not worth it. Casters can steal Mage and Priest gear (as long as they're not actually stealing it from actual Mages and Priests) if it's an upgrade, but Warriors almost never have a reason to slum it up in mail.