I run my Pal group with a shaman healer (Pal/Shm/Brd/Zerk/Rng/Wiz). They pair well as Pal gives you cleric style hp buffs, gives you a second battle-rezzer and a Pal's powerful self heals can make it easier for Sham in tough fights. Rng can be setup as your backup healer, which is one of the roles I have my Bst fill in their group.
Malo
Both Shm and Mage get the same Malo line of debuffs. They get an AA version, AE version and also have an un-resistable lower level spell version. You can run these in DPS as
debuffall |malo lines and KA will prioritize getting malo on every mob in camp. At higher levels, both classes also have a combo DPS/Malo spell that you can run early in the DPS sequence to get the debuff on without losing damage.
Idol of Malos is a lesser used Sham spell that creates an AE totem in camp that casts Malo debuff (-40 saves) on all nearby mobs. I find this super helpful for bard groups. You need to watch when you're running this at some camps though, the totem will aggro any nearby mobs including mobs above/below you on the z-axis.
DPS1=Idol of Malos|99|Once
Cleric is definitely the most versatile healer with the best insta-saves, but Sham does add more to a melee focused group. Clerics also have the best battle-rez in game, but if you have 2 rezzers in group that isn't as important.
Bst's buffs are basicly the same as Shm, though you miss out on the extra stuff from Shm's focus (+11% avoidance is a big one.) Bst's big contributions are a pet for back-up tank and full-group weapon delay reduction (haste v4) via their Dicho/Diss buff.