I've seen this twice, both times within the same guild and the guild has killed Heroic Garrosh. Could anyone explain to me why they are using the 2 Piece Elemental? I thought maybe one of them had logged off in the wrong gear, but when I found two people doing the same thing it got me wondering.
Resto 2 piece is pretty mandatory; it is typically 5%-9% extra output (which is easily viewable now that they changed the combat log in 5.4.2 to credit the healing to the Shaman). Resto 4 piece is pretty much garbage though; it is typically worth less than a 0.5% output increase and 3 of the 5 tier pieces have Mastery on them, making the itemization pretty undesirable. It is definitely not worth using Resto 4 piece if you have the option of instead using 2 piece + 2 heroic Warforged offset items. The better itemization of the offset items plus the extra stats from the 6 extra ilevels gives you more output than the actual set bonus does. Even using 2 regular heroic pieces without mastery on them is arguably more output than using the 4 piece (with the requirement to use at least 2 mastery pieces.).
As far as the Ele 2 pc; it's completely useless for Resto. It requires activating Fulmination stacks to get the DPS increase, and Resto doesn't get Fulmination. The only Ele tier piece that is worth considering is the shoulders in my opinion. All of the other pieces have Mastery on them. However, there are two different Heroic Warforged shoulder options (Protectors and Dark Shaman), both of which are going to beat Ele tier shoulders. The other thing to keep in mind is that they may be using Ele tier items simply because the offset pieces just won't drop. For example, I have yet to see the BiS Norushen legs drop in 15+ kills (even the non WF version), and that is on 25H. On 10H, the drop rate on offset items can be horrific. It's entirely possible they are just using the Ele tier items because Protector totems are rotting, it's obviously better than using ToT gear in those slots, the Resto 4 piece is useless and the Ele tier items are both better itemized and more easily interchangeable between the two specs.
If you are in no need of the spirit on the off-pieces and you play in a 10m guild, you will most likely be better off using the ele shoulders and chest, since the chances of getting the off-pieces WF are slim. And in my opinion ele shoulders and chest outperfrom the off-pieces.