It frees a Glyph slot, meaning another one for more dps.

Elemental Oath = Your non-periodic spell critical strikes grant your party or raid members within 100 yards Elemental Oath, increasing spell critical strike chance by 5%. In addition, while Clearcasting from Elemental Focus is active, you deal 10% more spell damage. Lasts 15 sec.

Up allways means bringing a Elemantal Shaman to the Raid will give you +5% spell crit for every caster (including priest). Not depending if the Shaman spells crit (they do most of the times), but from the start of the fight, for every spell.
Same for Clearcasting procs, because EO will be up allways, every CCP will allways give 10% more spell damage for the shaman.

Any idea how much more damage that is with a 10 / 25 man Raid?
That's a double buff.

And whyle the Elemantal Shaman would buff our spell casters, bring an Enhance Shaman for your melee.

Increases your expertise by 9, and causes your critical hits with melee attacks to increase all party and raid members' attack power by 10% if within 45 yards of the Shaman. Lasts 10 sec.

Forget about the 10 sec, it's allways up.
10% more AP for the whole raid during all the fights.

While any Raidleader would understood the value of both buffs, Shamans will be even more valuable to take along.

Although Restoration has some good Raid talents (mana tide totem, AA), compared to Ele / Enh they miss a kind off Raid wide aura. It made sense if they would have a x% more healing aura for the whole Raid, so every shaman tree would bring their own unique aura.