The shammy has a lot of totem buffs.
I see the group has a mix of melee (physical based) and casters (spell based), so this might be less of a bonus.

Putting one toon into a group... even when they're relatively undergeared...
If they bring something to the group that makes the others better.
That can be very worthwhile.

In my case, I went from four (elemental) shaman and a paladin (my PvE Heroic group for BC/WotLK).
And dropped one shammy, replacing her with a warlock.
The warlock did just under half of the DPS of the shaman she replaced.
But gave the group a 13% spell power boost.
Which resulted in a net DPS increase.

The shammy will bring melee/spell haste, spell power, etc.
Heroism/Bloodlust, is pretty big too.
Even the resistances (via glyph, on the Healing Stream totem), will greatly reduce the elemental type damages taken by the team.