For encounters with physical raid damage or random add aggro a shield is pretty damn useful. There are almost none of these encounters anymore. Pretty much everyone used the Halfus off-hand item last tier instead of the shield from Atremedes because the shield was itemized poorly, but because there are literally zero situations in T11 content with unavoidable physical raid damage that is mitigated by armor it doesn't matter at all.
Staves are a bit trickier, as you can't have the reserve option of grabbing your shield out of your bags mid-fight (at the very least you should still have a shield, even if it's the crafted stormshield or a PvP shield) unless you also have a reserve main-hand weapon. As far as I can tell, though, there are still no encounters in T12 where the armor of a raid member that isn't a tank actually matters, so it's a moot point.
As far as raw stats are concerned I'm pretty sure that a 391 Eye of Purification/397 Kogun, Hammer of the Firelord + 391 Ward of the Red Widow are going to be superior to any staff as long as you can get them without a problem. If you can't, sure, get a staff. It's not the end of the world. As far as the legendary goes, well, it's a 397 orange with an obscenely powerful proc, so if your guild actually thinks its a good idea to give it to an elemental shaman then it's obviously best in slot.