I think they should bring back Dungeon sets. I remember back durring vanilla ad BC where doint high level instances and heroics could get you a nice set of non-epic gear that got you ready to begin raiding, and also looked pretty cool too. It was nice to have have a good set of armor with set bonuses incase you missed out on getting tier that week. And if you had like the full D3 set, people would see you as committed and likely choose you for their raid rather than someone in a mix of blues greens and purples.
I remember when my hunter had the full D3 set durring BC and was able to pump out 1200+ dps, which was awsome back then for that gear. And I got into a really good raiding guild because of it and the only reason the picked me up was because I had the full D3 set with enchants and stuff and the crossbow from Kara and they were running SSC.
And having a full dungeon set rather than a bunch of crafted items and stuff showed that you had spent alot of time playing and should know your class well. And this was because, unless you were really lucky and got most of the set pieces to drop most the time, it usually took at least a few weeks to get that entire set because the bosses would only drop 1 or 2 pieces of D3 that covered several different classes and specs.
But this is just my opinion. If it doesn't happen I wouldn't care too much.