working for memory, so don't scream if I am off by two points...
Canni 3 is like 40 mana for 75 health, castable about once every 4 seconds, cast time + spell gem refresh.
SoD mana regen for max med is like 52... every 6 seconds.
Thus, a multicanni hot key ends up costing you a lot of health for extremely minor increase in mana over just sitting and medding. That's canni 3. I think you might actualy lose mana with Canni 1 and 2, but I do nto care to work out the med rate for those levels. Canni 4 was like 80 for 150, or double Canni3 for the same cast time.
So, yeah, if you are level 60, you can press the multicanni key, I guess.
Or, you could be half way competent and cast cannis in between med ticks. Its not something that can be macroed, but it changes a shaman for 50% effective to 100% effective. The only limitation of the shaman is mana, and statistic buffs are the least effective aspect, with the exception of DDs.
In a group, I expect a shaman to be crowd control, slower, resistant debuffer, off tank, and backup healer. If the group has the shaman as the only healer, then there is enough DPS in the group that I expect the shaman to be primary healer.