I agree with the point about every piece of gear can be used by another class except for SP plate (well there are SP dk's, lol). It just seems like a wasted drop. As far as making a new hero class to deal with this, Blizz could easily take out that gear and put the armor mastory for holy pallies as mail which would be a LOT easier to impliment at this point in the development of cata than creating a whole new class. Your idea for a new hero class kinda sounds like a melee priest imo. I'm not saying that it wouldn't be interesting to have in the xpac after cata, but to have a new class ready to go for cata would push back the release of cata and i'm ready for cata to be released yesterday and i'm sure many are too.
The same could be said for cloth armor, but in reverse. I've ran into people that believe that ANY gear with spirit on it is purely healer gear even though other classes benefit from spirit. Yet these people that say this as a healer also wear gear that doesn't have spirit. Thankfully spirit is being taken away in Cata.