Trauma is great for a raid healing resto Shaman, because Chain Heal spam applies a blanket of
Chained Heal and
Earthliving HoTs on several of the targets it affects, which in conjunction with the direct healing of the spell itself provides several chances of proccing Trauma.
Trauma tends to be rightfully regarded as being best in the hands of a Resto Druid because of its outstanding synergy with the Wild Growth ability. Each use of the Wild Growth ability causes 42 chances of proccing the ability, so this ability by itself accounts for the majority of a Rdruid's Trauma procs. Often, a Rdruid will find great use of this weapon even if the fight only intermittently involves AoE healing.
Holy Pallies aren't as suited to the weapon as the above 2 specs. Holy Priests don't really benefit from it quite as much either. Glyphed Prayer of Healing causes 15 chances of proccing Trauma (or more, since party members' pets are also healed), but PoH usage doesn't share the sustainability of Chain Heal or Wild Growth.