For a highly mobile group, such as Affliction Warlocks... a shammy has a lack of mobility.
The only real choices are Druid or Priest.

For a group with any kind of tank, by definition the tank will be tanking.
Which means you won't need to be mobile for the vast majority of the time.

But the shammy brings:
- Air (Melee Haste or Spell Protection or Nature Resist)
- Earth (Fear Protection or Strength & Agility)
- Fire (Spell Power for the Shammy or AoE damage or Cold Resist)
- Water (Passive AoE Heals or Fire Resistance or Disease/Poison cleansing)
- Bloodlust/Heroism for almost every boss.

Riptide is a decent heal on the move.
And Earth Shield on the tank will proc when hit, even if moving.

Chain Heal is not instant, but it is absurdly efficient and will hit everyone in melee (glyphed) or almost everyone otherwise; and it intelligently jumps from the initial target to the most hurt in order.
Plus you have a quick heal and a big heal, and Nature's Swiftness for an emergency heal.
And can self rez for wipe recovery.
And bring Water Walking (like Path of Frost) or Water Breathing.

For a group without a shaman in a dps role, a resto is a very strong option.
And any tank & spank group, isn't built on being mobile... ie read almost never stopping movement.

Play what you want...
It is your group.