I think it makes sense for hunters at least be able to add not only mail armor pieces to their collections but also leather armor as well. Perhaps this is already the case in Classic WoW, if it has Tmog options. Vanilla WoW, hunters would have to wear leather until a certain lvl…40 maybe…and then they would be able to wear mail armor! Sometimes i still see a hunter every once in a while mixing it up with leather and mail to their gear. Just a thought. 
Man it looks like noone is interested in wearing leather lol
They aren’t going to allow hunters (or any other class) to add appearances to their collection from more than one armor type.
I wouldn’t mind being able to apply leather appearances to my mail armor; I don’t like most of the mail armor sets. OTOH I haven’t bothered to go back and collect any set pieces on my leather wearing alts, so I don’t know if I’d actually like any of the leather sets better.
Thanks everyone for the feedback! 
